Plasma Catalysis: Fundamentals and Applications [1st ed. 2019] 978-3-030-05188-4, 978-3-030-05189-1

Table of contents :
Front Matter ....Pages i-ix
Plasma Catalysis: Introduction and History (J. Christopher Whitehead)....Pages 1-19
Plasma Catalysis Systems (Akira Mizuno, Michael Craven)....Pages 21-46
Plasma-Catalyst Interactions (Hyun-Ha Kim, Yoshiyuki Teramoto, Atsushi Ogata)....Pages 47-68
Plasma Catalysis Modeling (Annemie Bogaerts, Erik Neyts)....Pages 69-114
Plasma-Catalytic Removal of NOx in Mobile and Stationary Sources (Ahmed Khacef, Patrick Da Costa)....Pages 115-144
Plasma-Catalytic Removal of VOCs (Pieter Cools, Nathalie De Geyter, Rino Morent)....Pages 145-180
Plasma-Catalytic Decomposition of Ammonia for Hydrogen Energy (Yanhui Yi, Li Wang, Hongchen Guo)....Pages 181-230
Plasma-Catalytic Conversion of Methane (Tomohiro Nozaki, Seigo Kameshima, Zunrong Sheng, Keishiro Tamura, Takumi Yamazaki)....Pages 231-269
Plasma-Catalytic Conversion of Carbon Dioxide (Bryony Ashford, Yaolin Wang, Li Wang, Xin Tu)....Pages 271-307
Plasma-Catalytic Reforming of Alcohols (Dae Hoon Lee)....Pages 309-341
Plasma Catalysis: Challenges and Future Perspectives (J. Christopher Whitehead)....Pages 343-348

Citation preview

Springer Series on Atomic, Optical, and Plasma Physics 106

Xin Tu J. Christopher Whitehead Tomohiro Nozaki Editors

Plasma Catalysis

Fundamentals and Applications

Springer Series on Atomic, Optical, and Plasma Physics Volume 106 Editor-in-Chief Gordon W. F. Drake, Department of Physics, University of Windsor, Windsor, ON, Canada Series Editors James Babb, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA Andre D. Bandrauk, Faculté des Sciences, Université de Sherbrooke, Sherbrooke, QC, Canada Klaus Bartschat, Department of Physics and Astronomy, Drake University, Des Moines, IA, USA Charles J. Joachain, Faculty of Science, Université Libre Bruxelles, Bruxelles, Belgium Michael Keidar, School of Engineering and Applied Science, George Washington University, Washington, DC, USA Peter Lambropoulos, FORTH, University of Crete, Iraklion, Crete, Greece Gerd Leuchs, Institut für Theoretische Physik I, Universität Erlangen-Nürnberg, Erlangen, Germany Alexander Velikovich, Plasma Physics Division, United States Naval Research Laboratory, Washington, DC, USA

The Springer Series on Atomic, Optical, and Plasma Physics covers in a comprehensive manner theory and experiment in the entire field of atoms and molecules and their interaction with electromagnetic radiation. Books in the series provide a rich source of new ideas and techniques with wide applications in fields such as chemistry, materials science, astrophysics, surface science, plasma technology, advanced optics, aeronomy, and engineering. Laser physics is a particular connecting theme that has provided much of the continuing impetus for new developments in the field, such as quantum computation and Bose-Einstein condensation. The purpose of the series is to cover the gap between standard undergraduate textbooks and the research literature with emphasis on the fundamental ideas, methods, techniques, and results in the field. More information about this series at http://www.springer.com/series/411

Xin Tu • J. Christopher Whitehead Tomohiro Nozaki Editors

Plasma Catalysis Fundamentals and Applications

Editors Xin Tu Department of Electrical Engineering and Electronics University of Liverpool Liverpool, UK

J. Christopher Whitehead School of Chemistry The University of Manchester Manchester, UK

Tomohiro Nozaki Department of Mechanical Engineering Tokyo Institute of Technology Tokyo, Japan

ISSN 1615-5653 ISSN 2197-6791 (electronic) Springer Series on Atomic, Optical, and Plasma Physics ISBN 978-3-030-05188-4 ISBN 978-3-030-05189-1 (eBook) https://doi.org/10.1007/978-3-030-05189-1 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

The combination of nonthermal plasma and heterogeneous catalysis, known as plasma catalysis, is regarded as a promising and emerging technology for environmental cleanup, energy conversion, and the synthesis of fuels and chemicals at ambient pressure and low temperatures. In nonthermal plasmas, the working gas is activated to create highly energetic electrons and a range of reactive species (free radicals, excited atoms, ions, and molecules) for the initiation of plasma-assisted physical and chemical reactions. Nonthermal plasma has a distinct non-equilibrium character, which means the overall gas kinetic temperature in the plasma can be as low as room temperature, while the electrons are highly energetic with a typical energy of 1–10 eV. As a result, nonthermal plasma can easily break most chemical bonds and overcome the disadvantage of high temperature and/or high pressure required by conventional thermal catalysis and enable thermodynamically unfavorable chemical reactions to proceed at low temperatures under ambient conditions. The high reaction rates and fast attainment of steady state in plasma chemical reactions allows rapid start-up and shutdown of the process compared to thermal treatment technologies, which significantly reduces the overall energy cost and offers a promising route for chemical energy storage using renewable energy sources such as solar and wind power. The coupling of nonthermal plasma with suitable catalysts has great potential to reduce the activation energy of reactions, enhance the conversion of reactants, and improve selectivity toward the desired products. These all contribute in different ways to enhancing the energy efficiency of the plasma process, as well as the catalyst stability by reducing poisoning, coking, and sintering, due to a synergy that occurs between the plasma and the catalyst. Plasma catalysis has been investigated for the removal of low-concentration environmental pollutants in high-volume waste gas and liquid streams, for the synthesis of platform chemicals and synthetic fuels from a range of sources (e.g., methane, carbon dioxide, and biomethanol), and for the synthesis of carbon nanomaterials. Different plasma sources and plasma-catalyst configurations have been designed and developed to enhance and optimize the performance of plasma-catalytic processes. Plasma catalysis has also been extended v

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Preface

to the synthesis, preparation, and treatment of catalysts (e.g., supported metal catalysts) at low temperatures as an attractive alternative to the thermal processes (e.g., thermal calcination and reduction) for catalyst preparation. As we know, the interactions between plasma and catalyst are very complex in a hybrid plasma catalysis system, especially when using a single-stage plasma catalysis configuration where the catalyst is placed directly in the plasma. Both the properties of the plasma (e.g., local and global electric field) and catalyst (e.g., surface structure, morphology, metal active sites) can be modified by the presence of each other, which in turn affects the physicochemical interactions between the plasma and catalyst, consequently changing the performance of the chemical reaction. In situ diagnostics and plasma modeling have been identified as the key tools to better understand the reaction mechanisms on the catalyst surface under plasma environment. There have been considerable and increasing research activities in this emerging area in recent years. Plasma catalysis is a broad, highly interdisciplinary, and still evolving field. Covering even the most important aspects of plasma catalysis in a single book that reaches readers ranging from students to active researchers in academia and industry is a significant challenge. This book provides a unique opportunity for a group of world-leading researchers active in the field of plasma catalysis to work together and to contribute to the first book specified in plasma catalysis, covering both fundamentals and applications of plasma catalysis. This book is divided into three sections. The first section (Chaps. 1, 2, 3, and 4) of the book provides a broad overview of plasma catalysis history, plasma catalysis systems, plasma-catalyst interactions, and plasma catalysis modeling. The second section (Chaps. 5, 6, 7, 8, 9, and 10) of the book covers a wide range of plasma-catalytic chemical processes including the removal of NOx and volatile organic compounds (VOCs), ammonia decomposition for hydrogen production, methane activation, carbon dioxide conversion, and alcohol reforming. The last section (Chap. 11) discusses the challenges and future perspectives to the advance of this emerging field. We believe the broad range of topics included in this book will provide the reader with an insight into the realm of possibilities that now exist in plasma catalysis research and development, and bridge the gap between plasma and catalysis community. It is also our hope that this book will stimulate further research in this exciting area. We would like to express our deepest appreciation to all the authors who shared our excitement in preparing this book and contributed unique chapters that illustrate the depth and breadth of this emerging research topic. We gratefully acknowledge the assistance of our students Yaolin Wang, Michael Craven, and Jonathan Harding. Liverpool, UK Tokyo, Japan Manchester, UK

Xin Tu Tomohiro Nozaki J. Christopher Whitehead

Contents

1

Plasma Catalysis: Introduction and History . . . . . . . . . . . . . . . . . . J. Christopher Whitehead

1

2

Plasma Catalysis Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Akira Mizuno and Michael Craven

21

3

Plasma-Catalyst Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hyun-Ha Kim, Yoshiyuki Teramoto, and Atsushi Ogata

47

4

Plasma Catalysis Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Annemie Bogaerts and Erik Neyts

69

5

Plasma-Catalytic Removal of NOx in Mobile and Stationary Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Ahmed Khacef and Patrick Da Costa

6

Plasma-Catalytic Removal of VOCs . . . . . . . . . . . . . . . . . . . . . . . . 145 Pieter Cools, Nathalie De Geyter, and Rino Morent

7

Plasma-Catalytic Decomposition of Ammonia for Hydrogen Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Yanhui Yi, Li Wang, and Hongchen Guo

8

Plasma-Catalytic Conversion of Methane . . . . . . . . . . . . . . . . . . . . 231 Tomohiro Nozaki, Seigo Kameshima, Zunrong Sheng, Keishiro Tamura, and Takumi Yamazaki

9

Plasma-Catalytic Conversion of Carbon Dioxide . . . . . . . . . . . . . . . 271 Bryony Ashford, Yaolin Wang, Li Wang, and Xin Tu

10

Plasma-Catalytic Reforming of Alcohols . . . . . . . . . . . . . . . . . . . . . 309 Dae Hoon Lee

11

Plasma Catalysis: Challenges and Future Perspectives . . . . . . . . . . 343 J. Christopher Whitehead vii

Contributors

Bryony Ashford Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, UK Annemie Bogaerts Research group PLASMANT, Department of Chemistry, University of Antwerp, Antwerp, Belgium Pieter Cools Research Unit Plasma Technology, Department of Applied Physics, Ghent University, Ghent, Belgium Michael Craven Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, UK Patrick Da Costa Sorbonne Université, Institut Jean Le Rond d’Alembert, Saint Cyr l’école, France Nathalie De Geyter Research Unit Plasma Technology, Department of Applied Physics, Ghent University, Ghent, Belgium Hongchen Guo State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning, China Seigo Kameshima Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, Japan Ahmed Khacef GREMI, UMR 7344 CNRS – Université d’Orléans, Orléans, France Hyun-Ha Kim National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan Dae Hoon Lee Plasma Engineering Laboratory, Korea Institute of Machinery and Materials, Daejeon, South Korea Department of Environment & Energy Engineering, University of Science and Technology, Daejeon, South Korea viii

Contributors

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Akira Mizuno Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Toyohashi, Aichi, Japan Rino Morent Research Unit Plasma Technology, Department of Applied Physics, Ghent University, Ghent, Belgium Erik Neyts Research group PLASMANT, Department of Chemistry, University of Antwerp, Antwerp, Belgium Tomohiro Nozaki Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, Japan Atsushi Ogata National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan Zunrong Sheng Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, Japan Keishiro Tamura Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, Japan Yoshiyuki Teramoto National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan Xin Tu Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, UK Li Wang State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning, China College of Environmental Sciences and Engineering, Dalian Maritime University, Dalian, Liaoning, China Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, UK Yaolin Wang Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, UK J. Christopher Whitehead School of Chemistry, The University of Manchester, Manchester, UK Takumi Yamazaki Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, Japan Yanhui Yi State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning, China

Chapter 1

Plasma Catalysis: Introduction and History J. Christopher Whitehead

1.1

Historical Introduction

Plasma catalysis (sometimes called plasma-enhanced catalysis, plasma-catalyst coupling, plasma-assisted catalysis or plasma-driven catalysis) is a hybrid technique where a catalytic material is used in conjunction with a gas discharge yielding a viable technique which gives enhanced performance for a range of gas processing applications such as removal of pollutants such as NOx, SOx and volatile organic compounds (VOCs) and production of a range of chemicals such as ammonia from N2 and H2, hydrogen and oxygenates by the reforming of hydrocarbons and a range of added-value chemicals from the conversion of CO2 [1–9]. Perhaps in order to properly set the scene for discussing the mechanism and impact of plasma catalysis, it is necessary to separately examine the two techniques that were brought together to form this hybrid process. Heterogeneous catalysis in which a solid catalytic material is placed into a stream of reactive gases can provide alternative reaction pathways with lowered energy barriers. These can increase the rates of the reactions involved, increasing the overall yield of the process at a given temperature and thereby improving the efficiency of the process. Mechanistically, these alternative reaction pathways may also improve the yield or selectivity for particular products, and using different catalysts can favor particular outcomes. This technique, often called thermal catalysis, has been used for hundreds of years and now forms the basis of many large-scale industrial processes particularly in the oil, gas and chemical industries. Catalysis was discovered by Sir Humphry Davy in 1817 who noted that heated platinum gauze or foil would bring about the slow combustion of vapors such as alcohol, ether, coal gas and methane below their ignition temperature [10]. Davy conducted his research at the Royal Institution in London and is probably most remembered for his invention of a J. C. Whitehead (*) School of Chemistry, The University of Manchester, Manchester, UK e-mail: [email protected] © Springer Nature Switzerland AG 2019 X. Tu et al. (eds.), Plasma Catalysis, Springer Series on Atomic, Optical, and Plasma Physics 106, https://doi.org/10.1007/978-3-030-05189-1_1

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J. C. Whitehead

miners’ safety lamp. (See the very readable account of the life of Davy by Thomas [11] for more information.) The Swedish chemist, Berzelius, acknowledged Davy’s discovery in his treatise on catalysis where that word was first used. Berzelius recognized the existence of a catalytic force that ‘is reflected in the capacity that some substances have, by their mere presence and not by their own reactivity, to awaken activities that are slumbering in molecules at a given temperature’ [12]. We now most commonly use the definition given by Ostwald: “Catalysis is the acceleration of a chemical reaction, which proceeds slowly, by the presence of a foreign substance which is not in itself necessary for the reaction” [13]. It is a question of the kinetics rather than the energetics of the overall reaction. The study of gas discharges where a high voltage is applied to electrodes in a glass tube at reduced pressure had its origins at the start of the eighteenth century when Francis Hauksbee, an English draper, observed a glow in an evacuated tube containing mercury when the tube became electrically charged: electroluminescence [14]. It took another century before significant advances in gas discharges were obtained. This in part came from the development of better vacuum pumps such as that of Geissler in 1855. This allowed Geissler to make evacuated tubes with platinum electrodes containing different gases and show that discharges of different colors could be produced that were characteristic of the gas in the tube. These tubes were initially regarded as scientific novelties [15]. Michael Faraday, who worked at the Royal Institution in London, as Humphry Davy’s assistant, looked at the structures or strata of light that were produced in gaseous discharges noting that the glow was not uniform but was brighter close to the electrodes with a dark space in between [16]. In time, such observations led to an understanding of the identity of the species responsible for luminescence and conductivity in gaseous discharges. In 1834, Faraday also laid down the foundations for the mechanism of heterogeneous catalysis based on his observation of the spontaneous combination of oxygen and hydrogen on a platinum surface at room temperature [17]. He proposed that the two gases were condensed on the surface of the metal and that “the approximation of the particles to those of the metals may be very great”. He stated that there would be a continual interchange of particles between the adsorbed layer and the gas in contact with it. The water product was less attracted by the platinum than the reacting gases and so could evaporate. He emphasized the importance of the forces between the different species stating: “I admit . . . that the sphere of action of the particles extends beyond those other particles with which they are immediately and evidently in union, and in many cases produces effects arising into considerable importance” [18]. It could be said that the work of Davy and Faraday at the Royal Institution played a key role in establishing the foundations of the two components that would come together a century later to form the basis of plasma catalysis. Plasma is an ionized gas containing an equal number of positive ions as of electrons and negative ions. It is thus electrically neutral and its degree of ionization can range from very low, i.e. partially ionized, to 100%, or fully, ionized. In the laboratory or factory, plasma can be created by a variety of discharge techniques involving chambers with electrodes energized by direct dc, pulsed or ac currents including electrodes screened by a dielectric material or electrode-less systems such

1 Plasma Catalysis: Introduction and History

3

as inductively or capacitively coupled radio frequency radiation and microwaves. These discharges bring about a breakdown of the gas and create a range of species such as electrons, ions, dissociated and excited species giving plasma reactive properties that can potentially bring about chemical transformations, and which might be enhanced when a catalyst is present. Additionally, we can characterize plasma by the pressure regime in which it operates which can be either low, being less than atmospheric, typically ca. 100 mbar or high pressure which is atmospheric or higher. Working at pressures less than atmospheric requires the provision of vacuum pumps which adds to the complexity and cost of the system, but low pressure conditions favor surface collisions making catalytic effects easier to notice, and they also minimize the deactivation of the excited states produced in the plasma through gas-phase collisions but at the price of reduced gas throughput. Another distinction concerns the degree of thermal equilibrium achieved in the plasma. In thermal plasma, all the degrees of freedom including the electrons, ions and neutral species are equilibrated and have the same temperature as the bulk gas (typically >1000 K). Nonthermal plasma has a high degree of non-equilibrium between the light electrons and the heavier particles such as the ions, radical and molecules. The disparity in mass between the electron and the gaseous species means that little kinetic energy is transferred between them and the heavy atomic and molecular species remain close to their ambient temperature. Thus, a nonthermal discharge can create excited and reactive species that can only be produced in an equilibrium system such as an arc or flame at very high temperatures. This means that we can have an ionized, excited and reactive gas interacting with the catalyst at temperatures at which conventional thermal catalysis would be inactive. Typically, much of the research in plasma catalysis has focused on the use of atmospheric pressure, nonthermal plasma because of the simplification in engineering systems without the need for vacuum systems and its operation at low temperatures that minimize corrosion and deterioration of the catalyst through sintering or coking. It is unclear whether the discovery of the effect of combining a catalyst with plasma was a deliberate act or just a serendipitous observation. Kim [1] has given an extensive history of the development of plasma catalysis which pinpoints the first experiment where a catalyst was intentionally combined with an electrical discharge as being performed in 1921 by Ray and Anderegg [19]. They attempted to oxidize carbon monoxide to carbon dioxide by first using a silent discharge in an atmospheric pressure mixture of oxygen and carbon monoxide and then passing these gases over a silver catalyst. They noted that: “It is obvious that part of the oxidation takes place in the ozonizer while the gases are under the influence of the silent discharge and that part takes place while the gases are under the influence of the catalyst”. The use of a catalyst downstream gave enhanced oxidation of the CO compared to experiments performed in 1879 by Berthelot [20] who used a similar configuration but without a catalyst and attributed the incomplete CO oxidation to decomposition of the CO2 product back to CO [13]. Over the next three or four decades, reports were relatively infrequent and focused on systems of industrial interest where potential improvement in efficiency

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Fig. 1.1 The number of articles describing “plasma catalysis” published during different 5-year periods from 1960. Note that the period 2015–2019 only contains publication up to June 2018. (The data has been obtained by analysis using the Web of Science databases)

could be transformational. These included gas to liquid conversion of hydrocarbons, ammonia production and decomposition and the removal of VOCs. Experiments were performed both at atmospheric and reduced pressures where the latter has more potential for identifying any possible influence of the catalyst as the ratio of surface to gas-phase collisions increases as the gas pressure decreases. If we use the number of publications as an indicator of activity in the field of plasma catalysis, it would seem from Fig. 1.1 that research in the field took off in the 1990s, and, while still relatively modest, it is increasing rapidly.

1.2

The Plasma Catalysis Effect

There are two main configurations in which a catalyst can be combined with plasma. In one, the catalyst is place directly in contact with the plasma (variously called a one-stage arrangement or in-plasma catalysis, IPC) and, in the second, the catalyst is placed downstream of the discharge (a two-stage arrangement or post-plasma catalysis, PPC). These arrangements can be extended in a complex way by using a sequence of catalysts each with a different role. For example, the destruction of low concentrations toluene in air was achieved by initially treating the gas stream using a series of packed-bed discharges followed by two downstream catalysts in sequence [21]. These were MnO2 followed by a MnO2/CuO mixed catalyst. In the absence of the catalysts, the plasma only destroys a proportion of the toluene, but the addition of either of the catalysts results in complete removal. Oxidation of toluene gives CO2, but CO is also produced due to incomplete oxidation. The MnO2/CuO

1 Plasma Catalysis: Introduction and History

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catalyst promotes the conversion of CO to CO2. The MnO2 catalyst is effective in totally removing any excess ozone that is produced in the discharge from the breakdown of air and not used in the catalytic oxidation of the toluene. Compared to a thermal catalysis experiment where the energy needed to promote reaction comes from supplied heat, in plasma-activated catalysis the energy comes from the electrical discharge. This produces reactive and energetic species in the gas phase that may then come into contact with the surface of the catalyst. The primary species produced in plasma are generally short-lived and consist of electrons, photons, ions, excited-state atoms or molecules and radicals. In the one-stage plasma-catalyst arrangement, it will be these species that will interact with the catalyst. But in a two-stage plasma-catalyst configuration, it will only be the relatively long-lived species that exit from the plasma that are in contact with the downstream catalyst. These will be the end-products, by-products and long-lived reactive intermediates from the plasma processing (of which ozone is an important and common example, as noted above) and, possibly, vibrationally excited species. Vibrational energy can be a significant mode for efficiently activating surface adsorption and desorption processes. The effectiveness of plasma catalysis processing can be assessed in several ways. Commonly, the products of the processing are determined by analysis of the downstream gas either in real time or by taking samples for remote analysis. A variety of spectroscopic and analytical techniques are employed such as Fourier-transform infrared spectroscopy (FTIR), gas chromatography (GC) and mass spectrometry. The formation of liquid products can also be monitored by downstream condensation and collection. To be useful, the analysis of the end products should be performed in a quantitative manner although the extent of removal of the input gas can be achieved by a relative measurement. Qualitative measurement of the minor end products or intermediates, which aids their identification, may be of value in determining the reaction mechanism. The primary quantities that are measured include the degree of conversion or destruction, X, of the input species, AB, usually expressed as a percentage: X ð% Þ ¼

No: of moles of AB removed  100 Total no: of moles of AB

Another important quantity is the selectivity, S, for the formation of a particular product, CD, Sð % Þ ¼

No: of moles of CD formed  100 No: of moles of AB removed

where appropriate allowance must be made for the stoichiometry of the reaction. The prime consideration in the assessment of the effectiveness of the combination of plasma with catalyst is that there should be some improvement in either of these two parameters as a consequence of hybridizing the two techniques. One measure of this is to determine whether any synergy results from the combination. This can be evaluated using a synergy factor where the measured effect of the processing

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(e.g. the conversion or selectivity) using the combination of plasma and catalyst must be greater than the sum of the effect of processing using plasma alone and of catalyst alone under equivalent conditions [22]. The synergy factor will be greater than one when the combination is synergistic. This is illustrated in Fig. 1.2 from the work of Wang et al. [23] on the decomposition of NH3 in a dielectric barrier discharge reactor packed with a range of metal catalysts. Unfortunately, far too many publications mistakenly assume that the combination of plasma and catalysis will always be synergistic, but that is not necessarily true, and only the calculation of a synergy factor that is greater than unity will provide the evidence of synergy. The addition of the catalyst to a discharge may improve the conversion or product selectivity compared to using the discharge on its own, but that does not necessarily demonstrate a synergistic effect. Vandenbroucke et al. studied the plasma catalysis decomposition of the decomposition of trichloroethylene (TCE) and found that the synergy factor for the degree of dissociation of TCE ranges from 0.78 to 4.78 in a range of experiments with changing parameters demonstrating that synergy may only exist under certain experimental conditions in a particular plasma catalysis system [22]. Another very important criterion for judging the effectiveness of plasma catalysis is to consider the energy efficiency of the processing. This is of vital importance in objectively comparing one plasma catalysis process with another that may use a different catalyst or plasma configuration. It is also a way in which the technology of plasma catalysis can be compared with other competitive technologies and is a key way in which to judge the economics and hence the commercial viability of plasma catalysis as a processing technology for a particular application. Essentially, the energy efficiency tells us how much material we can convert or produce for a defined amount of energy. It can be expressed in a bewildering array of units, e.g. kg/J,

Fig. 1.2 Demonstration of the synergistic effect of plasma catalysis for the conversion of NH3 at a specific energy density of 28 kJ/L using a dielectric barrier reactor packed with a range of supported metal catalysts at a temperature of 450  C. Results are presented for plasma alone, catalyst alone and the plasma-catalyst combination. (Reproduced from Ref. [23] with permission. Copyright 2015, the American Chemical Society)

1 Plasma Catalysis: Introduction and History

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mol/J, molecule/eV, mol/kWh, etc., but they can be easily interconverted! The reciprocal of the energy efficiency is sometimes used and called the specific energy. Gutsol et al. have provided a very useful review with summaries of different ways of describing energy efficiency in plasma systems [24]. Alternatively, the energy efficiency can be expressed by comparing the actual energy required to create (or destroy) a given amount of material with the theoretical energy required for the same process. This theoretical energy for the formation or destruction process can be identified with the reaction enthalpy for that process if a clear reaction mechanism has been identified. In considering the performance of a plasma catalysis system, a balance often has to be struck between different outcomes. For example, it is not always possible to maximize conversion or yield and energy efficiency simultaneously. It is commonly found that as the energy supplied to the system (commonly represented as the specific energy density, SED, or the specific energy input, SEI, which is the energy input per unit volume of the reactor) increases, the conversion increases, but the energy efficiency decreases. Figure 1.3 demonstrates this for the decomposition of CO2 in a packed bed plasma reactor filled with BaTiO3. It can be seen that the optimal energy efficiency is obtained at the lowest SEI value, while the magnitude of the dissociation increases with increasing energy input. In Fig. 1.4, this effect is demonstrated by a plot that shows the optimum regions in terms of energy efficiency and the obtained decomposition of CO2 for a range of different plasma sources with and without catalysts [26]. Such a plot can be used as an aid to evaluate which plasma or plasma-catalyst combination will be best to achieve a particular performance. This methodology has been extended to a wider range of plasma systems for the conversion of pure CO2 and the dry reforming of CH4 with methane CO2 and demonstrates that certain plasma systems are more effective for certain processes. (See Figs. 24 and 32 in [8].) Fig. 1.3 A comparison of the variation of % CO2 conversion and the energy efficiency as a function of the SEI for the treatment of an atmospheric pressure stream of pure CO2 in a DBD reactor packed with BaTiO3 beads. (Drawn using data taken from Ref. [25])

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Fig. 1.4 Comparison of conversion and energy efficiency for CO2 decomposition with different types of nonthermal plasmas. Square symbols represent pure CO2; triangles, CO2-Ar mixture; inverted triangle, CO2-H2O mixture; diamond, CO2-H2O-Ar mixture. MW microwave, RF radio frequency, GAP gliding arc plasmatron, MW-ss microwave (supersonic flow), DBD dielectric barrier discharge. The SEI is given for reference. (Reproduced from Ref. [26] with permission. Copyright 2017, Elsevier)

1.3 1.3.1

How Does Plasma Catalysis Work? What Does the Plasma Do to the Catalyst?

It is not the aim of this section to give an answer to this question as that will be the focus of many of the chapters that follow. Instead, I will just illustrate the way in which researchers have attempted to rationalize the observation of a plasma catalysis effect. Essentially there are two basic ways in which plasma catalysis works as mentioned before. First, it could be that placing a catalyst either directly in a plasma or downstream from it changes the operation of the discharge in a physical or chemical way that enhances the processing. Alternatively, using plasma to activate a catalyst changes the behavior of the catalytic process in a beneficial way. Basic theories of catalysis tell us that there are several stages involved in a catalytic reaction [27]. Firstly, species in the gas phase must collide with the surface and become adsorbed. Following adsorption, they might migrate on the surface to a reactive site that might be located within a pore inside the catalyst. At this stage, surface reaction can take place between species adsorbed on the surface, which is called a Langmuir-Hinshelwood mechanism, or a species still in the gas phase may react with one that is adsorbed, which is designated an Eley-Rideal process. Then, the surface-bound reaction product must desorb from the surface and make its way back into the bulk gas.

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In thermal catalysis, the surface is heated to supply energy to overcome any barriers to adsorption. Species can be strongly adsorbed onto the catalyst with bonds that are comparable to chemical called chemisorption, or the bonding can be weaker in magnitude comparable to physical forces such as van der Waals and that is called physisorption. The nature of binding to the catalyst depends on its chemical composition giving the active sites for binding and the nature of the adsorbed species. Adsorption may be of the species in the same form as it is when gaseous or it might involve the dissociation into more reactive fragments, e.g. the dissociation of molecular hydrogen into two hydrogen atoms when adsorbed on Ni, a process called dissociative chemisorption. Adsorption of species onto a surface can improve reactivity by increasing their interaction time with the plasma, and this goes someway to explaining why enhanced performance can be obtained in plasma catalysis systems when using materials such as alumina, Al2O3 or titania, TiO2, that in conventional catalysis would be regarded as being inert support materials. It is important to realize that the rate of progress of a plasma catalysis process can be determined not only by the adsorption of the reagents, but also if a reaction product (or intermediate) is strongly adsorbed onto the surface and the reaction slows down as fewer surface sites become available for adsorption. The relative binding energies to the surface (enthalpies of adsorption) for the reagents, intermediates and products are key in determining the effectiveness of the catalytic process. As an example, using infrared in situ probing of the surface loading of a TiO2 catalyst during plasma catalysis, Barakat et al. [28] have investigated the oxidation of isopropanol by ozone in a downstream configuration and find that the rate of formation of gaseous end-products is limited by the rate of oxidation of an acetone intermediate, which saturates the surface. As we have previously mentioned, there is a rich wealth of species in the plasma that includes many species that are potentially reactive including ions, excited atomic and molecular species and molecular and atomic free radicals created from gas-phase dissociation that are all created in the discharge. Once created, these species can undergo gas-phase collisions that can create further unstable species by reaction or excitation. Alternatively, the collisions can bring about deactivation by quenching or by recombination. Some short-lived excited species will decay by emitting a photon, a process called fluorescence that in the case of electronically excited species yields ultraviolet or visible emission giving the characteristic color to the discharge as was observed by Hauksbee and Faraday in their nineteenth century experiments with gaseous discharges. In determining how these discharge-produced species might interact with the catalyst, it is not just necessary to consider how energetic or reactive they might be, but we must also consider their lifetimes in the gas phase that result from the collisional processes of activation, reaction and deactivation, just described and also how far they can travel during this lifetime. In other words, do these plasma-created species make it to the catalyst? Typically at atmospheric pressure and room temperature, they travel less than 100 nm and the time between collisions is less than 1 ns. The lifetime of a ground state oxygen atom, O(3P), in an atmospheric pressure plasma in dry air is about 14 μs [29] and that of a ground state OH radical depends

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on the gas composition but is typically 100 μs in an air plasma although this reduces to about 20 μs when a potential reagent such as trichloroethylene is added [30]. This means that there will only be a narrow region immediately above the catalyst surface from which the short-lived plasma-created species can be adsorbed. Kim et al. [31] have calculated that for ground state oxygen atoms or OH radicals, this layer will have a thickness of ~50 μm. Outside of this layer, species produced by the plasma will react or be deactivated in the gas phase in the same way that they do in the absence of the catalyst. This suggests that to gain the maximum effect from plasma catalysis, the reactor must be designed in a way that produces the reactive species very close to the surface of the catalyst. Many catalysts consist of a metal coated onto a support material, and the effectiveness of the catalyst depends not only on the identity of the metal but also on the fractional coverage of the support by the metal and the size of the deposited metal particles. In many experiments, it is found that using nanoscale particles increases their effectiveness. At a molecular level, the dynamics of the reactive processes involved in the plasma-catalytis processes are of many types. In Fig. 1.5, some possible processes for the formation of ammonia in hydrogen/nitrogen dielectric barrier plasma with a Ru metal catalyst deposited on an alumina support are illustrated [32]. These indicate the interplay between gas-phase excitation and dissociation processes and adsorption onto both the metal particle and the alumina support, spillover onto the alumina from the Ru and migration of species across the alumina surface, followed by surface reaction and desorption of products. Gaseous excited N2 molecules created in the discharge adsorb dissociatively onto the alumina, while H2 is either dissociated in the gas phase or on the Ru metal. Diffusion of N and H atoms from the metal across the surface of the alumina creates NH3 in a stepwise manner by two different reactions where the H atoms created on the Ru metal react more quickly with the N atoms on the Al2O3 than do the H atoms adsorbed directly from the gas phase. This of course is a mechanism proposed to explain kinetic measurements without any direct identification of species on the catalyst surface.

Fig. 1.5 Schematic diagram of the reaction pathways in the formation of NH3 from a N2/H2 plasma on a Ru/Al2O3 catalyst. (Taken from Ref. [32] by permission. Copyright 2007, Springer)

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Another important effect that the plasma may have on the catalyst is that of surface modification. Indeed, plasma is increasingly being used as a method of pre-treating catalysts for conventional thermal processing [33, 34]. For example, using a low pressure glow discharge or radio frequency plasma with Ar, He, N2 or air as a source of energetic electrons, it is possible to reduce a metal in a more environmentally sustainable way than with conventional thermal or chemical processing [35, 36]. Plasma preparation can change the dispersion of metals on a catalyst, allowing for the treatment of sensitive low temperature materials and the production of novel classes of catalysts involving selective doping and the production of nanoparticles. It seems clear that if plasma can be used to prepare a catalyst in isolation under controlled conditions, then with the normal operating conditions of a one-stage plasma catalysis process, some of the plasma activity may result in a continual dynamic modification of the catalytic surface. Marinov et al. [37] have noted this for a SiO2 surface under exposure to a low pressure nitrogen plasma commenting that the surface under contact with the plasma is not static and suggesting that this type of behavior is expected for a range of surfaces and plasma chemistries. This dynamic interaction between plasma and catalyst may give a unique benefit in terms of reduced poisoning or coking and increased stability and activity. Finally, we should consider how the catalyst and its adsorbed species will behave when exposed to the electrons produced by the discharge. Electron-stimulated desorption is a well-known process in surface science. This can arise due to local heating of the surface by electron impact leading to thermal desorption. It is also possible that the addition of electrons can affect the bonding between a molecular adsorbate and the surface creating repulsive interactions leading to desorption. Dissociative electron attachment of a species on the surface giving an anion that then dissociates may be an important process when dealing with electronegative species. It has been shown [38] that electron attachment to a CO2 molecule that is adsorbed onto an oxygen vacancy on a TiO2 surface brings about dissociation of the CO2 anion to give CO and the filling of the oxygen vacancy by the O ion. The CO may then desorb migrate to another region of the surface. Mei et al. [39] have developed this mechanism to describe the dissociation of CO2 on photocatalytic materials such as BaTiO3 and TiO2 where plasma-produced electrons of sufficient energy can create electron-hole pair states in an analogous method to the excitation of these materials by photons of the same energy in photocatalysis. This process is shown schematically in Fig. 1.6.

1.3.2

What Does the Catalyst Do to the Plasma?

In general terms, placing a catalytic material into plasma will affect the physical and electrical properties of the gas discharge that might thereby cause some modification to the outcomes of the plasma-catalytic processes. The addition of packing material might change the nature of the discharge from a filamentary form that propagates as

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Fig. 1.6 A schematic mechanism for the plasma catalysis conversion of CO2 on the surface of a photocatalyst. (Reproduced from Ref. [39] with permission. Copyright 2017, Elsevier)

microdischarges or streamers through the gas between the electrodes to that of a surface discharge that moves on the surface of the material. Such surface discharges on a catalytic surface have recently been imaged for the first time [40]. This change in discharge type may also be associated with an alteration of the electron energy distribution that may result in changes to the yields and identities of the species formed in the discharge. Neyts and Bal [41] has recently suggested that the addition of a catalytic material to a discharge can introduce an additional electric field influencing the retention time of the species at the catalyst surface and altering the charge distribution at the surface increasing reactivity. The efficiency of the processing will also depend on the structural features of the catalyst such as its morphology, porosity and chemical activity as these may interact with the nature of the plasma. The use of dielectric materials as packing can have a profound effect on the electrical properties of the discharge by changing the capacitance of the reactor. This can influence the breakdown voltage and the charge that can be transferred between the electrodes. In general, more electrical energy can be deposited into a discharge when materials of higher dielectric constant are used resulting in the formation of more electrons of higher energy giving increased yield of ionized and excited species. However, the overall effect of using high dielectric materials on the efficiencies and selectivities for the plasma catalysis processing is by no means clear and is found to vary between systems suggesting that there may be a subtle interplay between several effects that is not completely understood. Interesting observations have been made where a dielectric barrier discharge reactor is filled with spherical beads of a high dielectric material such as BaTiO3 or ZrO2. Such materials are often ferroelectric, and polarization effects can cause enhanced electric fields at the points of contact between the beads giving higher concentrations of excited species. The discharge transitions from streamers propagating in the gas when there is no packing to a mixture of gaseous and surface discharge when there is packing present. Increased conversion of CO2 is observed [25, 42–44] in these arrangements, and simulations [45, 46] confirm that there are enhanced electric fields where the beads touch and the electron density is higher in these regions. This causes stronger and faster development of the plasma. The bead size is also important in affecting the conversion of the CO2 and increases as the bead

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size decreases as long as it is possible to sustain a discharge as the breakdown voltage for plasma action increases strongly as the size decreases [44]. Michielsen et al. [43] suggest that all these observations have wider implications for plasma catalysis as the conversion obtained is not just a function of the active sites on the catalyst which is deposited on a support material but also on how that support materials is packed into the reactor. There is a positive contribution of the packing on the conversion obtained, due to electric field enhancement at the contact points, but a negative contribution due to the lower residence time of the gases because of the volume reduction at a given flow rate and a positive or negative effect depending on the nature of material inserted into the voids. This conclusion that the important role played by how a reactor is packed with catalyst is supported by observations that using a dielectric barrier discharge (DBD) reactor that is only partially filled with a catalyst doubles the CH4 conversion and H2 yield during the dry reforming of CH4 with CO2 compared to a filled reactor [47]. The nature of the catalyst surface can have an effect on the electric field in the discharge, and irregularities in the surface such as roughness or the presence of pores can create local variations in the electric field and local regions of field intensification that can become a source of high-energy electrons. The islands of nanosized dispersed metals on a support will also encourage enhanced electrical fields as will edges, steps or other irregularities in the crystalline structure of the catalyst. Pores can selectively adsorb molecules depending on their size increasing their reactivity, and penetration of plasma into the pores may specifically excite such species. Experimental studies on the removal of pollutants from air [48] show that for nanoporous materials (pore size 50 or for metallic coatings, the discharge was found to be more localized, due to very weak surface charging. Finally, it is worth to stress again that the PIC-MCC simulations reveal that microdischarges can be formed inside nm-sized pores, while the fluid model in previous section predicted that this was only possible for μm-sized pores. The reason is that the fluid model results were obtained for a helium discharge, operating in glow mode, as explained above, while the results presented here are for an air discharge, operating in filamentary mode, which is characterized by much higher electron densities in the streamers, and in this case, the plasma can thus be created even in nm-sized catalyst pores, as long as they are larger than the Debye length at these conditions.

4.4.3

Adsorption of Plasma Species on the Catalyst Surface, and Desorption of Newly Formed Species, that Might Affect the Plasma Chemistry

Plasma species that adsorb on the catalyst surface can give rise to the formation of new species upon reaction at the surface. When these new species desorb from the catalyst surface, they arrive back in the plasma. Hence, when modeling the plasma

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chemistry for plasma catalysis application, the desorbed species need to be included in the model as well. There exist several models for describing the plasma chemistry for various environmental applications relevant for plasma catalysis, e.g., NOx destruction [107–111], SO2 removal [112, 113], the oxidation of carbon soot [108, 110, 114], VOC remediation [115–119], and hydrocarbon reforming and/or CO2 conversion [120–133], but they are mostly applied to plasma without catalysis, and thus, the effect of desorbed species from a catalyst surface was typically not yet taken into account. Indeed, most often, 0D chemical kinetics models are used for this purpose, so that surface effects are typically not included at all. There exist also some (1D or 2D) fluid models for this application (e.g., [120, 125, 132, 133]), but they typically consider only simplified boundary conditions, based on sticking coefficients at the walls, thus not accounting for possible chemical reactions at a catalyst surface either. A few papers, however, report on the plasma chemistry, accounting also for the effect of a catalyst. Kim et al. developed a simple kinetic model for the plasma catalytic decomposition of VOCs, predicting zero-order kinetics with respect to the specific energy input, in good agreement with their experimental observations [83]. Tochikubo simulated NOx removal by plasma catalysis, based on a fluid model for a filamentary DBD, dealing with the plasma chemistry and a limited set of 17 surface reactions [81]. However, the synergy expected for plasma catalysis could not be observed, as the plasma simulations were not directly coupled to the surface reaction modeling. Moreover, the authors stated that the input data (activation barriers, rate constants) will need to be improved for better correspondence with experiments. Istadi and Amin developed an artifical neural network for a catalytic DBD reactor for dry reforming of methane, suggesting some synergism between the plasma and the CaO-MnO/CeO2 catalyst, which affects the selectivity toward hydrocarbons with two or more C atoms [134]. Finally, Jiwu and Lei modeled the flue gas desulfurization process by a corona discharge combined with Mn2+ catalysis [135]. The Mn2+ catalyst was however in liquid phase, and thus no surface reactions were included. There is still a lot of work to be performed in this field, but the latter can only be done when the surface reaction probabilities are known, pointing out the need of more atomic-scale simulations to obtain information on the surface reactions in case of plasma catalysis (see Sect. 4.3 above). When such reaction coefficients become available, the above plasma chemistry models could be upgraded by including the effects of a catalyst surface, through appropriate boundary conditions for the species continuity equations, so that these models, preferably in 2D, become effectively applicable to plasma catalysis.

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Conclusion and Outlook: Ultimate Goal for Modeling Plasma Catalysis

The ultimate goal for modeling plasma catalysis will be to include all the above effects into one comprehensive, multi-level model. This includes describing the dynamic behavior at the atomic and molecular level, which takes place at very short time scales (order of nanoseconds), up to the slower bulk kinetic behavior, as well as mass and heat transfer, taking place at macroscopic time scales. It is clear that such a model has not yet been realized. In other application fields, however, like for plasma etching and deposition, surface effects have been incorporated already. More specifically, the effect of surface reactions has been accounted for by updating the plasma chemistry with new species formed at the walls (e.g., [136–141]). Also the effect of surface temperature on the temperature in the plasma, which could also be relevant for plasma catalysis in case of hot spots (see Sect. 4.3.2 above), has been described already [142]. Typical codes which account for such effects are 2D hybrid models, like the Hybrid Plasma Equipment Model (HPEM) [141] and non-PDPSIM (e.g., [97–99]), both developed by Kushner and coworkers. The non-PDPSIM code has already been applied for multi-scale fluid-surface kinetics modeling of plasma treatment of rough polymer surfaces, which can be considered comparable to a porous catalyst surface (see Sect. 4.4.2 above). Furthermore, the HPEM code also allows to calculate the formation and evolution of trench profiles due to etching and the behavior of plasma species inside trenches, by means of a Monte Carlo simulation [143]. Such features would also be of great value for the simulation of plasma species inside catalyst pores. It is thus clear that such a hybrid model would also be of great value for plasma catalysis applications. This concept is illustrated in Fig. 4.25. As plasma catalysis applications typically entail a comprehensive plasma chemistry, the most suitable type of model for this purpose is a 0D (or global) model, because of its reasonable computational cost, even when including a large number of species and chemical reactions. However, such a model does not account for geometrical (reactor) effects. Therefore, once the plasma chemistry is built up, it should be transferred to 2D or 3D fluid models, which account for geometrical effects, like in a packed bed DBD reactor, or surface effects, to allow updating the plasma chemistry by new species formed at the catalyst surface. Because such a model is computationally very expensive, the plasma chemistry will have to be based on a reduced chemistry set, which can be developed within the 0D chemical kinetics model, by comparing and benchmarking with the full chemistry model. Such a 2D or 3D model also calculates the electric field, which might affect the plasma-catalyst interactions. Furthermore, it provides the fluxes of the various plasma species arriving at the catalyst surface, which can subsequently be used to describe the behavior of plasma species inside a catalyst pore, e.g., by another fluid model or a Monte Carlo model (cf. Sect. 4.4.2 above). Ideally, the latter should also be able to account for changes in the pore shape, similar to what has been developed already for plasma etching applications (see above).

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Fig. 4.25 Concept of an integrated model, which would be the ultimate goal for modeling plasma catalysis

This combined, hybrid model should furthermore be combined with atomic-scale simulations for the interaction of the plasma species with the catalyst surface. Indeed, the fluxes of the various plasma species, as well as the electric field near the catalyst surface, calculated by the plasma model, are useful input for atomicscale simulations. Combining macro-scale plasma models and micro-scale models inside catalyst pores with atomic-scale simulations for the plasma-catalyst interactions is very challenging, due to the small time and length scales of the atomistic simulations, i.e., typically in the order of nanoseconds and nanometers, respectively [46]. Therefore, it is more realistic to use such simulations as a kind of off-line module to the plasma model, providing rate coefficients for the various surface reactions. The latter can then be used as boundary conditions for the plasma species continuity equations in the plasma model, to update the plasma chemistry. This will also allow adding new (desorbed) species from a catalyst surface to the plasma chemistry. In this way, the atomistic simulations can be integrated in a plasma model, yielding a multi-level model for plasma catalysis (see Fig. 4.25). It is clear that developing such an integrated model will require a lot of efforts, especially because of the large number of plasma-catalyst interactions that need to be accounted for. Hence, it cannot be realized on a short time scale. Nevertheless, we should aim for such a model, which would certainly contribute to a better understanding of plasma catalysis and can help to improve this highly important and rapidly evolving application field. In the meantime, until such a comprehensive model is being developed, the individual modeling approaches, as described in Sect. 4.3 and 4.4 above, can also contribute already to a better understanding of plasma catalysis.

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Acknowledgments We gratefully acknowledge K. Van Laer, Y.-R. Zhang, Y. Zhang, Q.-Z. Zhang, W. Wang, S. Huygh, and M. Shirazi (University of Antwerp) and M. Kushner (University of Michigan) for providing some of the figures used as illustrations in this chapter. The authors also acknowledge financial support from the Research Council of the University of Antwerp (TOP-BOF project), the IAP/7 (Interuniversity Attraction Poles) program “Physical Chemistry of PlasmaSurface Interactions (PSI)” by the Belgian Federal Science Policy Office (BELSPO), and the Fund for Scientific Research Flanders (FWO, grant no. G.0217.14 N).

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52. Neyts, E. C., & Ostrikov, K. (2015). Nanoscale thermodynamic aspects of plasma catalysis. Catalysis Today, 256, 23–28. 53. Neyts, E. C. (2016). Plasma-surface interactions in plasma catalysis. Plasma Chemistry and Plasma Processing, 36, 185–212. 54. Neyts, E. C., Ostrikov, K., Sunkara, M. K., & Bogaerts, A. (2015). Plasma catalysis: Synergistic effects at the nanoscale. Chemical Reviews, 115, 13408–13446. 55. Kim, H. H., Teramoto, Y., Negishi, N., & Ogata, A. (2015). A multidisciplinary approach to understand the interactions of nonthermal plasma and catalyst: A review. Catalysis Today, 256, 13–22. 56. Kim, H. H., Teramoto, Y., Ogata, A., Takagi, H., & Nanba, T. (2016). Plasma catalysis for environmental treatment and energy applications. Plasma Chemistry and Plasma Processing, 36, 45–72. 57. Neyts, E. (2018). Atomistic simulations of plasma catalytic processes. Frontiers of Chemical Science and Engineering, 12, 145–154. 58. Somers, W., Bogaerts, A., van Duin, A. C. T., & Neyts, E. C. (2014). Interactions of plasma species on nickel catalysts: A reactive molecular dynamics study on the influence of temperature and surface structure. Applied Catalysis B: Environmental, 154-155, 1–8. 59. Somers, W., Bogaerts, A., van Duin, A. C. T., & Neyts, E. C. (2012). Plasma species interacting with nickel surfaces: Toward an atomic scale understanding of plasma-catalysis. Journal of Physical Chemistry C, 116, 20958–20965. 60. Somers, W., Bogaerts, A., van Duin, A. C. T., Huygh, S., Bal, K. M., & Neyts, E. C. (2013). Temperature influence on the reactivity of plasma species on a nickel catalyst surface: An atomic scale model. Catalysis Today, 211, 131–136. 61. Huygh, S., Bogaerts, A., & Neyts, E. C. (2016). How oxygen vacancies activate CO2 dissociation on TiO2 anatase (001). Journal of Physical Chemistry C, 120, 21659–21669. 62. Huygh, S., & Neyts, E. C. (2015). Adsorption of C and CHx radicals on anatase (001) and the influence of oxygen vacancies. Journal of Physical Chemistry C, 119, 4908–4921. 63. Shirazi, M., Neyts, E. C., & Bogaerts, A. (2017). DFT study of Ni-catalyzed plasma dry reforming of methane. Applied Catalysis B: Environmental, 205, 605–614. 64. Bal, K. M., Huygh, S., Bogaerts, A., & Neyts, E. C. (2018). Effect of plasma-induced surface charging on catalytic processes: Application to CO2 activation. Plasma Sources Science and Technology, 27, 024001. 65. Bal, K. M., & Neyts, E. C. (2018). Modelling molecular adsorption on charged or polarized surfaces: A critical flaw in common approaches. Physical Chemistry Chemical Physics, 20, 8456–8459. 66. Neyts, E. C., & Bal, K. M. (2017). Effect of electric fields on plasma catalytic hydrocarbon oxidation from atomistic simulations. Plasma Processes and Polymers, 14, e1600158. 67. Neyts, E. C., van Duin, A. C. T., & Bogaerts, A. (2012). Insights in the plasma-assisted growth of carbon nanotubes through atomic scale simulations: Effect of electric field. Journal of the American Chemical Society, 134, 1256–1260. 68. Neyts, E. C., Ostrikov, K., Han, Z. J., Kumar, S., van Duin, A. C. T., & Bogaerts, A. (2013). Defect healing and enhanced nucleation of carbon nanotubes by low-energy ion bombardment. Physical Review Letters, 110, 065501. 69. Somers, W., (2015). Atomic scale simulations of the interactions of plasma species on nickel catalyst surface. University of Antwerp, PhD-thesis. 70. Ni, B., Lee, C., Sun, R.-C., & Zhang, X. (2009). Microwave assisted heterogeneous catalysis: Effects of varying oxygen concentrations on the oxidative coupling of methane. Reaction Kinetics and Catalysis Letters, 98, 287–302. 71. Zhdanov, V. P. (1999). Simulation of surface restructuring and oscillations in CO-NO reaction on Pt(100). The Journal of Chemical Physics, 110, 8748–8753. 72. Kersten, H., Deutsch, H., Steffen, H., Kroesen, G. M. W., & Hippler, R. (2001). The energy balance at substrate surfaces during plasma processing. Vacuum, 63, 385–431.

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73. Li, S., Zheng, W., Tang, Z., & Gu, F. (2012). Plasma heating and temperature difference between gas pellets in packed bed with dielectric barrier discharge under natural convection condition. Heat Transfer Engineering, 33, 609–617. 74. Nozaki, T., & Okazaki, K. (2013). Non-thermal plasma catalysis of methane: Principles, energy efficiency, and applications. Catalysis Today, 211, 29–38. 75. JiangB, G. H. (2016). Enhanced dissociative chemisorption of CO2 via vibrational excitation. The Journal of Chemical Physics, 144, 091101. 76. Bal, K. M., & Neyts, E. C. (2015). Merging metadynamics into hyperdynamics: Accelerated molecular simulations reaching time scales from microseconds to seconds. Journal of Chemical Theory and Computation, 11, 4545–4554. 77. Guerra, V., & Marinov, D. (2016). Dynamical Monte Carlo methods for plasma-surface reactions. Plasma Sources Science and Technology, 25, 045001. 78. Marinov, D., Teixeira, C., & Guerra, V. (2017). Deterministic and Monte Carlo methods for simulation of plasma-surface interactions. Plasma Processes and Polymers, 14, 1600175. 79. Blaylock, D. W., Ogura, T., Green, W. H., & Beran, G. J. O. (2009). Computational investigation of thermochemistry and kinetics of steam methane reforming on Ni(111) under realistic conditions. Journal of Physical Chemistry C, 113, 4898–4908. 80. Blaylock, D. W., Zhu, Y.-A., & Green, W. H. (2011). Computational investigation of the thermochemistry and kinetics of steam methane reforming over a multi-faceted nickel catalyst. Topics in Catalysis, 54, 828–844. 81. Toshikubo, F. (2009). Modeling for plasma-enhanced catalytic reduction of nitrogen oxides. Thin Solid Films, 518, 957–961. 82. Delagrange, S., Pinard, L., & Tatibouët, J.-M. (2009). Combination of a non-thermal plasma and a catalyst for toluene removal from air: Manganese based oxide catalysts. Applied Catalysis. B, Environmental, 68, 92–98. 83. Kim, H. H., Ogata, A., & Futamura, S. (2005). Atmospheric plasma-driven catalysis for the low temperature decomposition of dilute aromatic compounds. Journal of Physics D: Applied Physics, 38, 1292–1300. 84. Van Laer, K., & Bogaerts, A. (2016). Fluid modelling of a packed bed dielectric barrier discharge plasma reactor. Plasma Sources Science and Technology, 25, 015002. 85. Takaki, K., Chang, J.-S., & Kostov, K. G. (2004). Atmospheric pressure of nitrogen plasmas in a ferroelectric packed bed barrier discharge reactor. Part I. Modeling. IEEE Transactions on Dielectrics and Electrical Insulation, 11, 481–490. 86. Russ, H., Neiger, M., & Lang, J. E. (1999). Simulation of micro discharges for the optimization of energy requirements for removal of NOx from exhaust gases. IEEE Transactions on Plasma Science, 27, 38–39. 87. Babaeva, N. Y., Bhoj, A. N., & Kushner, M. J. (2006). Streamer dynamics in gases containing dust particles. Plasma Sources Science and Technology, 15, 591–602. 88. Kruszelnicki, J., Engeling, K. W., Foster, J. E., Xiong, Z., & Kushner, M. J. (2017). Propagation of negative electric discharges through 2-dimensional packed bed reactors. Journal of Physics D: Applied Physics, 50, 025203. 89. Kang, W. S., Kim, H. H., Teramoto, Y., Ogata, A., Lee, J. Y., Kim, D. W., Hur, M., & Song, Y. H. (2018). Surface streamer propagations on an alumina bead: Experimental observation and numerical modelling. Plasma Sources Science and Technology, 27, 015018. 90. Zhang, Y., Wang, H.-Y., Jiang, W., & Bogaerts, A. (2015). Two-dimensional particle-in cell/ Monte Carlo simulations of a packed-bed dielectric barrier discharge in air at atmospheric pressure. New Journal of Physics, 17, 083056. 91. Gao, M.-X., Zhang, Y., Wang, H.-Y., Guo, B., Zhang, Q.-Z., & Bogaerts, A. (2018). Mode transition of filaments in packed-bed dielectric barrier discharges. Catalysts, 8, 248. 92. Van Laer, K., & Bogaerts, A. (2017). Influence of gap size and dielectric constant of the packing material on the plasma behaviour in a packed bed DBD reactor: A fluid modelling study. Plasma Processes and Polymers, 14, e1600129.

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93. Van Laer, K., & Bogaerts, A. (2017). How bead size and dielectric constant affect the plasma behaviour in a packed bed plasma reactor: A modelling study. Plasma Sources Science & Technology, 26, 085007. 94. Wang, W., Kim, H.-H., Van Laer, K., & Bogaerts, A. (2018). Streamer propagation in a packed bed plasma reactor for plasma catalysis applications. Chemical Engineering Journal, 334, 2467–2479. 95. Michielsen, I., Uytdenhouwen, Y., Pype, J., Michielsen, B., Mertens, J., Reniers, F., Meynen, V., & Bogaerts, A. (2017). CO2 dissociation in a packed bed DBD reactor: First steps towards a better understanding of plasma catalysis. Chemical Engineering Journal, 326, 477–488. 96. Uytdenhouwen, Y., Van Alphen, S., Michielsen, I., Meynen, V., Cool, P., & Bogaerts, A. (2018). A packed-bed DBD micro plasma reactor for CO2 dissociation: Does size matter? Chemical Engineering Journal, 348, 557–568. 97. Bhoj, A. N., & Kushner, M. J. (2006). Multi-scale simulation of functionalizationof rough polymer surfaces using atmospheric pressure plasmas. Journal of Physics D: Applied Physics, 39, 1594–1598. 98. Bhoj, A. N., & Kushner, M. J. (2008). Repetitively pulsed atmospheric pressure discharge treatment of rough polymer surfaces: I. Humid air discharges. Plasma Sources Science & Technology, 17, 035024. 99. Bhoj, A. N., & Kushner, M. J. (2008). Repetitively pulsed atmospheric pressure discharge treatment of rough polymer surfaces: II. Treatment of micro-beads in He/NH3/H2O and He/O2/ H2O mixtures. Plasma Sources Science & Technology, 17, 035025. 100. Wang, X. M., Foster, J. E., & Kushner, M. J. (2011). Plasma propagation through porous dielectric sheets. IEEE Transactions on Plasma Science, 39, 2244–2245. 101. Zhang, Y.-R., Van Laer, K., Neyts, E. C., & Bogaerts, A. (2016). Can plasma be formed in catalyst pores? A modeling investigation. Applied Catalysis B: Environmental, 185, 56–67. 102. Zhang, Y.-R., Neyts, E. C., & Bogaerts, A. (2016). Influence of the material dielectric constant on plasma generation inside catalyst pores. Journal of Physical Chemistry C, 120, 25923–25934. 103. Zhang, Y.-R., Neyts, E. C., & Bogaerts, A. (2018). Enhancement of plasma generation in catalyst pores with different shapes. Plasma Sources Science and Technology, 27, 055008. 104. Zhang, Y., Wang, H.-Y., Zhang, Y.-R., & Bogaerts, A. (2017). Formation of microdischarges inside a mesoporous catalyst in dielectric barrier discharge plasmas. Plasma Sources Science and Technology, 26, 054002. 105. Zhang, Q.-Z., & Bogaerts, A. (2018). Propagation of a plasma streamer in catalyst pores. Plasma Sources Science and Technology, 27, 035009. 106. Zhang, Q.-Z., Wang, W.-Z., & Bogaerts, A. (2018). Importance of surface charging during plasma streamer propagation in catalyst pores. Plasma Sources Science and Technology, 27, 065009. 107. Gentille, A. C., & Kushner, M. J. (1995). Reaction chemistry and optimization of plasma remediation of NxOy from gas streams. Journal of Applied Physics, 78, 2074–2085. 108. Dorai, R., & Kushner, M. J. (2000). Consequences of propene and propane on plasma remediation of NOx. Journal of Applied Physics, 88, 3739–3747. 109. Dorai, R., & Kushner, M. J. (2003). Consequences of unburned hydrocarbons on microstreamer dynamics and chemistry during plasma remediation of NOx using dielectric barrier discharges. Journal of Physics D: Applied Physics, 36, 1075–1083. 110. Dorai, R., & Kushner, M. J. (2002). Repetitively pulsed plasma remediation of NOx in soot laden exhaust using dielectric barrier discharges. Journal of Physics D: Applied Physics, 35, 2954–2968. 111. Teodoru, S., Kusano, Y., & Bogaerts, A. (2012). The effect of O2 in a humid O2/N2/NOx gas mixture on NOx and N2O remediation by an atmospheric pressure dielectric barrier discharge. Plasma Processes and Polymers, 9, 652–689. 112. Chang, M. B., Balbach, J. H., Rood, J. J., & Kushner, M. J. (1991). Removal of SO2 from gas streams using a dielectric barrier discharge and combined plasma photolysis. Journal of Applied Physics, 69, 4409–4417.

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113. Chang, M. B., Kushner, M. J., & Rood, M. J. (1992). Removal of SO2 and the simultaneous removal of SO2 and NO from simulated flue gas streams using dielectric barrier discharge plasmas. Plasma Chemistry and Plasma Processing, 12, 565–580. 114. Chang, M. B., Kushner, M. J., & Rood, M. J. (1992). Gas-phase removal of NO from gas streams via dielectric barrier discharges. Environmental Science & Technology, 26, 777–781. 115. Storch, D. G., & Kushner, M. J. (1993). Destruction mechanisms for formaldehyde in atmospheric temperature plasmas. Journal of Applied Physics, 73, 51–55. 116. Evans, D., Rosocha, L. A., Anderson, G. K., Coogan, J. J., & Kushner, M. J. (1993). Plasma remediation of trichloroethylene in silent discharge plasmas. Journal of Applied Physics, 74, 5378–5386. 117. Aerts, R., Tu, X., De Bie, C., Whitehead, J. C., & Bogaerts, A. (2012). An investigation into the dominant reactions for ethylene destruction in non-thermal atmospheric plasmas. Plasma Processes and Polymers, 9, 994–1000. 118. Aerts, R., Tu, X., Van Gaens, W., Whitehead, J. C., & Bogaerts, A. (2013). Gas purification by nonthermal plasma: A case study of ethylene. Environmental Science & Technology, 47, 6478–6485. 119. Vandenbroucke, A. M., Aerts, R., Van Gaens, W., De Geyter, N., Leys, C., Morent, R., & Bogaerts, A. (2015). Modeling and experimental study of tricholoroethylene abatement with a negative dirrect current corona discharge. Plasma Chemistry and Plasma Processing, 35, 217–230. 120. De Bie, C., Martens, T., van Dijk, J., Paulussen, S., Verheyde, B., & Bogaerts, A. (2011). Dielectric barrier discharges used for the conversion of greenhouse gases: Modeling the plasma chemistry by fluid simulations. Plasma Sources Science and Technology, 20, 024008. 121. Yang, Y. (2003). Direct non-oxidative methane conversion by non-thermal plasma: Modeling study. Plasma Chemistry and Plasma Processing, 23, 327–346. 122. Pringle, K. J., Whitehead, J. C., Wilman, J. J., & Wu, J. H. (2004). The chemistry of methane remediation by a non-thermal atmospheric pressure plasma. Plasma Chemistry and Plasma Processing, 24, 421–434. 123. Agiral, A., Trionfetti, C., Lefferts, L., Seshan, K., & Gardeniers, J. G. E. (2008). Propane conversion at ambient temperatures C–C and C–H bond activation using cold plasma in a microreactor. Chemical Engineering and Technology, 31, 1116–1123. 124. Pinhao, N. R., Janeco, A., & Branco, J. B. (2011). Influence of helium on the conversion of methane and carbon dioxide in a dielectric barrier discharge. Plasma Chemistry and Plasma Processing, 31, 427–439. 125. De Bie, C., Verheyde, B., Martens, T., van Dijk, J., Paulussen, S., & Bogaerts, A. (2011). Fluid modelling of the conversion of methane into higher hydrocarbons in an atmospheric pressure dielectric barrier discharge. Plasma Processes and Polymers, 8, 1033–1058. 126. Aerts, R., Martens, T., & Bogaerts, A. (2012). Influence of vibrational states on CO2 splitting by dielectric barrier discharges. Journal of Physical Chemistry C, 116, 23257–23273. 127. Snoeckx, R., Aerts, R., Tu, X., & Bogaerts, A. (2013). Plasma-based dry reforming: A computational study ranging from the nanoseconds to seconds time scale. Journal of Physical Chemistry C, 117, 4957–4970. 128. Snoeckx, R., Setareh, M., Aerts, R., Simon, P., Maghari, A., & Bogaerts, A. (2013). Influence of N2 concentration in a CH4/N2 dielectric barrier discharge used for CH4 conversion into H2. International Journal of Hydrogen Energy, 38, 16098–16120. 129. Snoeckx, R., Zeng, Y. X., Tu, X., & Bogaerts, A. (2015). Plasma-based dry reforming: Improving the conversion and energy efficiency in a dielectric barrier discharge. RSC Advances, 5, 29799–29808. 130. Snoeckx, R., Heijkers, S., Van Wesenbeeck, K., Lenaerts, S., & Bogaerts, A. (2016). CO2 conversion in a dielectric barrier discharge plasma: N2 in the mix as a helping hand or problematic impurity? Energy and Environmental Science, 9, 999–1011. 131. Heijkers, S., Snoeckx, R., Kozák, T., Silva, T., Godfroid, T., Britun, N., Snyders, R., & Bogaerts, A. (2015). CO2 conversion in a microwave plasma reactor in the presence of N2: Elucidating the role of vibrational levels. Journal of Physical Chemistry C, 119, 12815–12828.

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132. De Bie, C., van Dijk, J., & Bogaerts, A. (2015). The dominant pathways for the conversion of methane into oxygenates and syngas in an atmospheric pressure dielectric barrier discharge. Journal of Physical Chemistry C, 119, 22331–22350. 133. De Bie, C., van Dijk, J., & Bogaerts, A. (2016). CO2 hydrogenation in a dielectric barrier discharge plasma revealed. J. Phys. Chem, C120, 25210–25224. 134. Istadi, A., & Amin, N. A. S. (2007). Modelling and optimization of catalytic-dielectric barrier discharge plasma reactor for methane and carbon dioxide conversion using hybrid artificial neural network - genetic algorithm technique. Chemical Engineering Science, 62, 6568–6581. 135. JiwuL, L. F. (2013). Modeling of corona discharge combined with Mn2+ catalysis for the removal of SO2 from simulated flue gas. Chemosphere, 91, 1374–1379. 136. Tinck, S., Bogaerts, A., & Shamiryan, D. (2011). Simultaneous etching and deposition processes during the etching of silicon with a Cl2/O2/Ar inductively coupled plasma. Plasma Processes and Polymers, 8, 490–499. 137. Tinck, S., De Schepper, P., & Bogaerts, A. (2013). Numerical investigation of SiO2 coating deposition in wafer processing reactors with SiCl4/O2/Ar inductively coupled plasmas. Plasma Processes and Polymers, 10, 714–730. 138. Tinck, S., Boullart, W., & Bogaerts, A. (2011). Modeling Cl2/O2/Ar inductively coupled plasmas used for silicon etching: Effects of SiO2 chamber wall coating. Plasma Sources Science and Technology, 20, 045012. 139. Kushner, M. J. (1987). A phenomenological model for surface deposition kinetics during plasma and sputter deposition of amorphous hydrogenated silicon. Journal of Applied Physics, 62, 4763–4772. 140. Zhang, D., & Kushner, M. J. (2000). Mechanisms for CF2 radical generation and loss on surfaces in fluorocarbon plasmas. Journal of Vacuum Science and Technology A, 18, 2661–2668. 141. Kushner, M. J. (2009). Hybrid modelling of low temperature plasmas for fundamental investigations and equipment design. Journal of Physics D: Applied Physics, 42, 194013. 142. Tinck, S., Tillocher, T., Dussart, R., & Bogaerts, A. (2015). Cryogenic etching of silicon with SF6 inductively coupled plasmas: A combined modelling and experimental study. Journal of Physics D: Applied Physics, 48, 155204. 143. Hoekstra, R. J., Grapperhaus, H. J., & Kushner, M. J. (1997). Integrated plasma equipment model for polysilicon etch profilesin an inductively coupled plasma reactor with subwafer and superwafer topography. Journal of Vacuum Science and Technology A, 15, 1913–1921.

Chapter 5

Plasma-Catalytic Removal of NOx in Mobile and Stationary Sources Ahmed Khacef and Patrick Da Costa

5.1

Introduction

Nitrogen oxides, which refer specifically to NOx (NO and NO2), are the major pollutants in the atmosphere from industries (mobile or stationary sources), which lead to acid rain and photochemical smog, and have been shown to be detrimental to human health and the environment. Automobiles and other mobile sources contribute to about 50% of the NOx that is emitted in the atmosphere, and nitrogen monoxide (NO) is the major portion of it [1, 2]. Lean-burn engines are receiving increasing application because of their promise of improved fuel economy over stoichiometric conditions. However, they offer special challenges for meeting increasingly stringent emissions standards because of the difficulty of removing NOx in an oxidizing environment where traditional three-way catalysts cannot work. Consequently, several methods collectively called deNOx processes have been developed to treat the effluents of mobile and power plants sources. Among them, selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are currently used to convert NOx into nitrogen (N2) molecules [3, 4]. Nowadays, the selective catalytic reduction of NOx by urea (urea-SCR) and lean-NOx trap technology (LNT) are the two most efficient NOx reduction technologies currently available for diesel engine emission control [5–8]. Both technologies provide high NOx reduction performance over a wide temperature range (200–400
C) under wellcontrolled conditions but still have serious drawbacks [9–19].

A. Khacef (*) GREMI, UMR 7344 CNRS – Université d’Orléans, Orléans Cedex 02, France e-mail: [email protected] P. Da Costa Sorbonne Université, Institut Jean Le Rond d’Alembert, Saint Cyr l’école, France e-mail: [email protected] © Springer Nature Switzerland AG 2019 X. Tu et al. (eds.), Plasma Catalysis, Springer Series on Atomic, Optical, and Plasma Physics 106, https://doi.org/10.1007/978-3-030-05189-1_5

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As an alternative to catalytic processes which require high temperatures nonthermal plasma (NTP) also referred as “non-equilibrium plasma” or “cold plasma,” such dielectric barrier discharges (DBDs) and corona discharges (CD) have been extensively investigated in the field of pollution control and frequently proposed in the literature for the removal of volatile organic compounds (VOCs), NOx, and SO2 [20–26]. In NTP, background gaseous species are chemically excited or dissociated directly by electronic impact, while the temperature of the reactants (i.e., gas temperature) remains relatively low, and thus the product distributions far from the chemical equilibrium may be obtained. In that case, the most useful deposition of energy is associated with the production of excited species (atoms, molecules) and activated species (radicals, ions) that eventually lead to the chemical conversion of pollutants. Although NTPs present attractive properties (low temperature, atmospheric pressure, compactness, low cost) and a unique way to induce gas-phase reactions by electron collisions, the formation of unwanted by-products and poor energy efficiency are serious obstacles toward their industrial implementation. To overcome these drawbacks, a more effective use of NTP is possible by exploiting its inherent synergetic potential through combination with heterogeneous catalysts as emphasized by different groups [27–29]. This innovative technique called plasma catalysis that combines the advantages of both NTP and catalysis has become a hot topic over the last decade. Catalyst can be combined with plasma in two ways: in-plasma catalysis (IPC), with the catalyst directly into the discharge zone, or postplasma catalysis (PPC), with the catalyst downstream the discharge zone. The conventional NTP reactors that are widely used for various environmental applications are subdivided according to the type of discharge mode (pulse, DC, AC, RF, microwave), presence of a dielectric barrier or catalyst, and geometry (cylinder, plane). It is important to note that the chemical potential of each discharge mode differs enormously from one discharge to another. Roughly speaking, the efficiency of a plasma discharge to remove pollutant from gas stream depends mainly on its ability to produce large amount of active species in the plasma. In an attempt to overcome the inherent shortcomings of these existing technologies, plasma- assisted catalysis for lean-NOx reduction through the improvement of the hydrocarbons (HC)- SCR technology has emerged as a promising alternative to the more mature urea-SCR and LNT technologies [30–37].

5.2

State of the Art on Catalytic NOx Abatement Systems

Since 1970, NOx abatement technologies were majorly based on selective catalytic reduction with ammonia (NH3-SCR). Nowadays, two mature technologies are used at least for mobile sources: NOx trap or NOx storage/reduction (NSR) and NH3-SCR [5–8]. Other promising technologies such as HC/HCR are still studied in order to use the reducing agent present in the feed for reducing the NOx in situ. The NOx trap is one of the actual technologies already in used in automotive posttreatment lines. The used materials are based on noble metal dispersed on a

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support such as alumina or ceria zirconia containing an adsorbent material such as barium oxide. Noble metal can be deactivated or poisoned since sulfur is present in the feed. The deactivation mechanisms of NSR catalysts, especially the sulfur poisoning and thermal degradation, were already extensively reviewed. Finally, recent developments of NSR catalysts were addressed in detail, concentrating on the improvements over precious metals, NOx storage materials, and metal oxide supports. Despite the first-generation NSR catalysts being quite successful in NOx emission control, new-generation NSR catalysts with low cost, high efficiency, and durability are urgently needed to meet the ever rigorous NOx emission regulations and develop the NSR technology. Thus, the development of new materials (e.g., perovskite oxide-based materials) [38] as alternatives of precious metals is very promising to reduce the cost of NSR catalysts. Finally, a novel synthesis method and a novel structured NSR catalyst (e.g., 3-D structured catalysts) would help to increase the NSR activity, sulfur poisoning resistance, and thermal stability needed for the NSR regeneration [39]. The SCR of NOx with NH3 was also considered for a long time to be the most efficient technology for reducing NOx emission in the presence of excess oxygen [40]. The V2O5-WO3(MoO3)/TiO2 material has been commercially employed as a NH3-SCR catalyst for a number of years. However, several serious problems with this catalyst still remain, e.g., the narrow temperature window (i.e., only applicable in 300–400
C), the high activity of SO2 oxidation, and the toxicity of V2O5 [41]. Therefore, new catalysts with environmentally benign characteristics and high SCR performance in a wide temperature range are required. Since the discovery of Cu-ZSM-5 catalyst as an efficient catalyst for NOx removal in 1986 by Iwamoto et al. [42], copper- or iron-exchanged zeolites have been and are still widely investigated [43]. However, the major disadvantage of zeolite-based materials is their poor resistance to high temperature treatment. New zeolitic systems such as SSZ-13 and SAPO-34 are in development to give to such system high SCR performance even after hydrothermal aging [44–47]. For SCR of NOx with HC, according to Burch et al. [5], more than 1000 catalysts were tested in laboratory, and more than 80% NOx conversion was obtained over specific temperature ranges under lean-burn conditions. It was never possible to translate this laboratory success to real exhaust automotive systems. The authors proposed three factors in order to explain that point: (i) Laboratory tests mostly focused on pure reducing agents such as the lower alkanes and alkenes, whereas onboard reducing agents, such as gasoline or diesel, contain hundreds of compounds, some of which may poison or inhibit the SCR reaction. (ii) The space velocities used in laboratory micro-reactors are frequently much lower than those found in real exhausts. (iii) The temperature range in an engine tests is much wider than the narrow window of high activity observed in typical laboratory tests.

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Thus, coupled plasma catalysis technologies could be a solution to the points (i) and (iii).

5.3

Nonthermal Plasma Reactors for NOx Treatment

Nonthermal plasmas for chemical treatment of industrial exhaust gases may be produced by a variety of electrical discharge reactors (corona discharge, surface discharge, dielectric-barrier discharge, etc.) [48–51] or by electron beam irradiation [52]. The electron beam technique, which has been first used, is based on irradiation of the gas flow by high energetic electrons being generated in ultrahigh vacuum accelerators with voltages ranging from tens of kV to some MV and transmitted into the gas flow through titanium foils. Each of the primary beam electrons generates a large number of secondary electrons in ionizing collisions, which have the right energy for efficient radical generation finally resulting in efficient pollutant removal. For some industrial and domestic applications, the pulsed corona and DBD reactors are much more suited than e-beam devices because of their high selectivity, moderate operating conditions (atmospheric pressure and room temperature), and relatively low maintenance requirements resulting in relatively low energy costs of the pollutant treatment.

5.3.1

Corona Reactors

Corona discharge is a transient luminous discharge that appears in regions of high electric field near sharp points, edges, or wires in electrically stressed gases prior to the point of electrical breakdown. The corona discharges are always nonuniform: a strong electric field, ionization, and luminosity are located in the vicinity of one electrode. The corona can be observed, for example, around high-voltage transmission lines, around lightning rods, along wire surrounded by a coaxial cylinder, or near irregularities in the form of sharp points, on the surface of a conductor at high voltage. The main physical and engineering principles of the corona discharges can be found in Loeb [53], Fridman et al. [54], and Rutgers and van Veldhuizen [55]. Various reactor configurations are used to generate corona discharges. In laboratory studies of corona discharges, the most common geometry is a pin-plane geometry (Fig. 5.1), where a needle is placed above a grounded plane. The high voltage is applied to the needle electrode. However, for industrial applications, this geometry is not sufficient, as it does not fill the whole gas volume with the discharge. The most popular geometries are the multi-pins-plane, wire- cylinder, and the saw-blade geometries [56, 57] as in electrostatic precipitators. The wire- cylinder geometry is probably used the most. It ensures a quite homogeneous distribution of the discharge that maximizes the active discharge volume and is easy to implement

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a

b

119

c

High Voltage Electrode

High voltage HV Generator (DC, AC, or pulsed) Ground Electrode

Fig. 5.1 Common corona reactor geometries: (a) pin-plane, (b) wire-cylinder, and (c) saw-blade

in a gas flow system. Often, multiple wire-cylinder reactors are mounted in parallel with regard to the gas flow to enable high gas throughput. Corona discharges are separated into two different categories: continuous and pulsed. Continuous corona discharges occur at DC or low-frequency AC voltages. A recent example of work on DC-excited corona discharges is given by Eichwald et al. [58]. Application of the continuous coronas is limited by low current and power, which results in a low treatment rate of exhaust gas. Increasing the corona power could result in a large current short circuit, and a large amount of NOx can be produced when spark breakdown occurs. For the purpose of gas cleaning, spark breakdown should be avoided. Increasing the corona power without transition to sparks becomes possible by using voltage pulses having nanosecond or sub-microsecond duration. Since discharge gaps of several tens centimeters can be used, the pulsed corona discharge (PCD) reactor is ideal for treatment of very large gas flow rates [59–61]. The costs of the PCD reactor are low compared to a DBD reactor for the same gas flow rate. However, the development of cost-effective pulsed power supplies is challenging. Further, for good efficiency and long lifetime of the power supply, care has to be taken for good electrical matching between power supply and reactor [62]. PCD reactors have been considered for the removal of pollutants both in the gas phase [48, 60, 63] and in the liquid phase [64]. A combination of pulsed corona with catalysts can be practical for applications to achieve improved results in the treatment of automotive exhausts and for hydrogen production from heavy hydrocarbons. Another interesting technological hybrid is related to the pulsed corona coupled with water flow. Such a system can be arranged either in the form of a shower, which is called a spray corona, or with a thin water film on the walls, which is usually referred to as a wet corona. Figure 5.2 shows an example of pulsed corona discharges developed from each pin in pure N2 at atmospheric pressure for a multi-pins-plane corona reactor. The illuminating paths indicate the position where the streamer heads have moved. Below a certain applied voltage, the corona discharge filaments are almost invisible to the human eyes, and only at the optimum voltage, which is not sufficient to generate arc discharge, and after a long adaptation in the dark, they can just be observed. The practical advantages of the PCD are that the short duration of the pulse ensures that no transition to spark takes place; therefore, it can be used at voltages

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Fig. 5.2 Streamer discharges in 12 mm gap in N2 at atmospheric pressure with a 16 kV pulse, 30 ns rise time, and half-width of about 80 ns (multi-pins-plane corona reactor) [63]

and currents higher than that at continuous corona can be used. Large voltages lead to higher electric fields and electron temperatures, which increases ionization and dissociation rates. Additionally, due to the short pulse duration, only electrons are significantly accelerated in few or tens of nanoseconds, and the gas heating can be kept to a minimum.

5.3.2

Dielectric Barrier Discharge Reactors

Another approach that avoids spark formation in streamer channels and instabilities in space (both effects are undesired in a volume discharge for chemical conversions) is based on the use of at least one electrically insulating barrier between the electrodes. The introduction of a dielectric barrier between the electrodes limits the direct current. Such discharges are referred to as dielectric barrier discharges or silent discharges (SD). The electrical circuit of the discharge reactor can be represented by the dielectric barrier capacity being in series with the discharge gap represented by the variable ohmic impedance of the gas discharge plasma paralleled by the capacitance of the discharge. Because of the capacitive coupling of the current to the discharge gap, DBD reactors have to be operated with AC or pulse repetitive voltages. Further reliable operation requires high-quality, low dielectric loss barrier materials having a high dielectric breakdown strength (>10 kV mm1) and high bulk resistivity (>108 Ω cm1). In case of low temperatures, polymeric barrier materials such as PTFE can be used. For exhaust gases having temperatures higher than 100
C, rather expensive densely sintered ceramics or quartz glass barriers are required. An advantage of DBD reactors is that barriers having certain catalytic properties can be used for plasma-catalytic hybrid treatment of gases. DBD processing is a very mature technology, first investigated by Siemens [65] in the 1850’s for ozone generation. Ozone formation requires three criteria: (i) energetic electrons that are able to efficiently dissociate O2 (4–5 eV, see Fig. 5.4), (ii) high pressure because of the three-body reaction which is responsible for ozone production, and (iii) low gas temperature because of the reduced lifetime of ozone at high gas temperature. DBDs can deliver all these requirements as the

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bulk gas temperature can be maintained close to room temperature (for high power, cooling the electrodes is required), while the electron temperature is typically 2–5 eV. DBDs are now routinely used in different industrial and fundamental applications such as water purification, polymer treatment, UV light generation, biological and medical treatment, pollution control, and exhaust cleaning from CO, NOx, SOx, and VOCs. DBDs could be generated in parallel-plane or in coaxial cylindrical reactor geometries, very common in ozonizers and other cases of gas treatment, as illustrated in Fig. 5.3. The flexibility of DBD configurations with respect to geometrical shape, operating medium, and operating parameters (input power, frequency, and gas flow) is remarkable. In many situations, discharge conditions optimized in small laboratory experiments can be scaled up to large industrial installations. Kogelschatz, Eliasson, and their group at ABB [67–69] have greatly contributed to the fundamental understanding and industrial applications of DBDs. Typical characteristics of the DBD micro-discharges in a 1 mm gap in atmospheric air are summarized in Table 5.1. Atmospheric pressure DBDs usually are filamentary. However, in certain gas mixtures, a quasi- homogeneous glow mode can be observed [70, 71]. Since the plasma-chemical efficiency of DBD reactors decreases with increasing discharge gap, typical discharge gap widths are in the range of a few millimeters only. When pulsed excitation with sub-microsecond rise time is applied, the discharge gap of DBD reactors can be increased from typically 1 mm to about 1 cm without loss in plasma-chemical efficiency [72]. Nevertheless, the treatment of industrial-scale gas flows requires a lot of paralleled discharge gaps in order to keep the flow resistance low.

Fig. 5.3 Common dielectric barrier discharge electrode configurations: (a–c) planar and (d) cylindrical

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Table 5.1 Characteristic of DBD in air at atmospheric pressure

5.4

Duration Filament radius Peak current Current density Total charge

109–108 s About 104 m 0.1 A 106–107 A m2 1010–109 C

Electron density Mean electron energy Filament temperature

1020–1021 m3 1–10 eV Close to average gas temperature in the gap

NOx Chemistry in Nonthermal Plasma

The energetic electrons produced in the plasma discharge mainly initiate the plasma chemistry. The elementary process of radical formation and reactions in NTPs can be broadly divided into a primary process and a secondary process based on the timescale of streamer propagation [73]. The primary process that has a typical timescale in the range of tenths of nanoseconds includes ionization, excitation, dissociation, light emission, and charge transfer. The efficiency of the primary process is highly dependent on energization methods and their parameters, such as nature of the power source (pulse, DC, or AC), voltage rise time, duration, and frequency. The secondary process is the subsequent chemical reactions involving the products of primary processes (electrons, radicals, ions, and excited molecules) to form additional radical species and reactive molecules (O3, HO2, and H2O2) by radical-neutral recombination. The secondary process is usually completed within approximately milliseconds. Oxidation is dominant for plasma processing of exhausts (with or without hydrocarbons) containing dilute concentrations of NOx (mainly NO with concentrations ranged from few ppm to hundreds ppm) in N2-O2H2O mixtures, particularly when O2 concentration is 5% or higher.

5.4.1

Plasma Without Hydrocarbons

In NTP treatment of NO-N2-O2-H2O mixtures, the kinetic energy of the electrons is mainly deposited into the major gas components N2, O2, and H2O. The result is the generation of N and O radicals through electron-impact dissociation:

e þ O2 ! e þ O 3 P þ O 3 P, 1 D

e þ N2 ! e þ N 4 S þ N 4 S, 2 D where O(3P) and O(1D) are the ground-state and metastable excited-state atomic oxygen radicals, respectively, and N(4S) and N(2D) are ground-state and metastable

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excited-state atomic nitrogen radicals, respectively. The N(4S) is the only plasmaproduced species that could effectively lead to the chemical reduction of NO [66, 74] accomplished via:
N 4 S þ NO ! N2 þ O The O radical resulting from previous reaction can lead to the oxidation of NO to NO2. However, the amount of O radicals is, as most, equal to the number of N(4S) produced in the plasma. Under this condition, most of the NO react with N(4S), and only a very small amount of NO2 is produced [75]. Most experimental investigations focused on the oxidation of NO to NO2 with different gaseous mixtures and energization systems. It was found that the oxidation of NO to NO2 significantly depends on gaseous mixtures. It is very important to make a distinction between NO removal by chemical oxidation and NO removal by chemical reduction. Oxygen amount in the gas mixtures could greatly suppress the reduction process and enhance the oxidation process or the opposite. Yan et al. [76] show that the reduction process-induced NO removal can be negligible in the presence of O2 with concentration of 3.6% of higher, which means that NO is completely converted into NO2. In that case, the oxidation pathway becomes dominant for two reasons: (a) The dissociation energy of O2 (4.8 eV) is lower than that of N2 (9.2 eV). For common atmospheric pressure electrical discharge plasma, the average electron kinetic energy is relatively low (3–6 eV) [77–79]. Under this condition, the rate for electron-impact dissociation of O2 is much higher compared to that of N2 as shown in Fig. 5.4. (b) High electron energies are required to optimize the production of N(4S) atoms by electron-impact dissociation of N2. Under conditions optimum for the dissociation of N2, a large number of excited nitrogen atoms, N(2D), is produced [80]. The N(2D) species can lead to undesired reactions in the presence of O2. Rather than reducing NO to N2, the N(2D) species would react with O2 to produce NO. If oxygen concentration is less than 1%, the reduction process is dominant as showed in pulsed corona discharge under high specific input energy (SIE > 30 Wh/m) [81]. In that case, the amount of NO2 produced could be larger than the removed NO because of N(2D)-induced NO formation and NO to NO2 conversion. The back conversion of NO2 to NO becomes a limiting factor for the NO oxidation efficiency and energy cost. As reported by Tas et al. [82], the energy required for this process ( 100 eV/molecule) is one order of magnitude higher than that for the oxidation process (50 eV/molecule) in the gas-phase reaction. Additionally, the secondary radical species such as HO2, NO3, and O3 are produced and consumed on longer timescales than the primary radicals. Ozone generated at low temperature through three-body reaction of O(3P) (O(1D) in humid gas) with O2 is quickly quenched by H2O to form hydroxyl radicals OH

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Percent of Input Power

40

N2 ionization

N2 vibrational N2 dissociation

30

20

O2 dissociation

10

O2 ionization 0 0

10

20

30

40

Electron Mean Energy (eV)

Fig. 5.4 Electrical power dissipation in a dry air discharge, showing the percent of input power consumed in the electron-impact processes leading to vibrational excitation, dissociation, and ionization of N2 and O2 [66]

which play an important role in the chemistry of NTP. As the oxygen and water vapor concentrations were increased, the rate of generation of OH and HO2 radicals by reactions of atomic oxygen with H2O molecules increases [22, 83]. These radicals (which density is highly sensitive to plasma conditions) initiate the reactions leading to chemical conversion of pollutant molecules (e.g., NO to NO2 and NO2 to nitric acid (HNO3)). Figure 5.5 shows the production of oxidizing agents (O, OH, and O3) was predicted by a self-consistent 0D-model in N2-O2-H2O mixture at room temperature and atmospheric pressure [84]. O and OH maximum densities are achieved in some tens of nanoseconds, whereas the density of ozone slowly increases to reach a maximum about 1 ms after the plasma ignition. In an oxygen-free mixture, the chemical kinetic is easier to study, but the situation is not representative of the engine or power plant exhaust gas emissions. The kinetic model in photo- triggered discharges of Fresnet et al. [85] predicts that N2(a’) singlet metastable states (which are strongly mixed by collisions and highly populated) represent an important pathway for NO removal in N2/NO mixtures, besides NO reduction by N atoms. The drastic decrease in NO removal observed experimentally in the presence of water was attributed to de-excitation of the N2(a’) states by the H2O molecules. For pulsed corona discharge in similar mixture, the kinetic model of Zhao et al. [86] considered reactions of NO and NO2 with N and O atoms and with excited N2(A3Σ) molecules and found that N2(A3Σ) molecules are important as well, especially for the conversion of N2O to N2.

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Fig. 5.5 O, OH, and O3 densities as a function of time predicted by a 0D selfconsistent model of phototriggered discharge: N2-O2H2O (77.6-20-2.4) at 25
C, 1 bar, and SIE ¼ 100 J/L [84]

5.4.2

Plasma with Hydrocarbons

It is well known that, related to NTP deNOx, promotion in the NO oxidation can be achieved with the addition of hydrocarbons. Hydrocarbon addition leads to substantial saving in the energy cost for deNOx treatment. Among the possible hydrocarbons, ethylene (C2H4) [87–89], propene (C3H6) [1, 22, 87, 89, 90], and propane (C3H8) [87, 90] were selectively investigated to estimate their usefulness for the NO-NO2 conversion. With the addition of hydrocarbons, the atomic oxygen produced in the plasma is considered to be the main initiator of the hydrocarbon chemistry, and the reactions typically produce intermediates such as hydroxy (OH) and peroxy radicals RO2 (with R ¼ H or CxHy). The chemistry of NTP with hydrocarbons is complex and not fully understood. In the literature, there are some recommendations according to the channels and the products of the reaction. As it was remarked in the experimental work of Tsang [91], the total rate constant of the C3H6 + O reaction is quite accurate, and the most serious uncertainties are in the branching ratios. Usually, authors use different reaction channels and branching ratios in the modeling. Penetrante et al. [1], Shin and Yoon [87], and Park et al. [92] included the data of Tsang [91] and Wilk et al. [93] in the oxidation chemical kinetic. The effect of C3H6 on the conversion pathways of NOx in pulsed corona discharge reactor was examined, and results reported by Penetrante et al. [1] show that in the very early stages of reaction, the propene is mainly consumed by O atoms following the three reaction channels: C3 H6 þ O !C2 H5 þ HCO !CH2 CO þ CH3 þ H !CH3 CHCO þ H þ H However, after the initial stages of the reaction chain, and following the production of hydroxyl radicals in the above reaction, the OH radical rather than the O atom

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becomes the main propene consuming species. The reaction products from these steps form hydrocarbon intermediates (CHO, CH3O, CH2OH, etc.) that react with oxygen to form HO2 radicals. Chemical kinetic analysis shows that HO2 radicals are the main species responsible for conversion of NO to NO2. The main end products predicted by the mechanism of Martin et al. [94] are CO, CO2, formaldehyde (CH2O), and acetaldehyde (CH3CHO), with smaller concentrations of ketene (CH2CO) and methyl ketene (CH3CHCO). Dorai and Kushner [90, 95, 96] used a recommendation of Atkinson [97] to develop a mechanism in humid exhaust gas. The reaction of O with C3H6 they used differs from those suggested by Penetrante et al. [1] by the inclusion of the steps: C3 H6 þ O !C3 H6 O !CH3 CH2 CHO !CH2 CHO þ CH3 !C2 H5 þ HCO For the reaction of OH with propene, only the following step is considered: C3 H6 þ OH ! C3 H6 OH End products such as formaldehyde (CH2O), acetaldehyde (CH3CHO), methyloxirane (C3H6O), and propanal (CH3CH2CHO) were predicted as major by-products from propene-induced NO to NO2 conversion. Experimental analysis of exhaust within similar composition made by Hoard and Panov [98] showed no presence of methyloxirane and propanal. In the work of Filimonova et al. [89], the choice of reaction channels and their ratios was based on the experimental investigation of branching ratios in the C3H6 + O reaction obtained by Koda et al. [99]. Taking into account the measured yields of products and a pressure dependence of CH2CHO formation, they used the following distribution: C3 H6 þ O !CH3 þ CH2 CHO !C2 H5 þ CHO !C2 H4 þ CH2 O Regarding these three significant examples, an ambiguity about the choice of critical reaction channel (C3H6 + O) for NOx oxidation needs to be clarified by additional experimental investigations and further refinement of the rate constants. In most cases, the comparison between experimental and simulation results is not suitable because the data is obtained in different conditions. However, some of the end products predicted by simulation results such as acetaldehyde, formaldehyde, methanol, methyl nitrate and nitrite, nitromethane, propylene oxide, and formic and nitric acids agree with experimental measurements as shown in the Figs. 5.6 and 5.7.

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Fig. 5.6 Typical FTIR spectra produced by plasma processing (27 J/L) at room temperature of (a) C3H6 (500 ppm)-N2, (b) NO (500 ppm)-C3H6 (500 ppm)-N2, and (c) O2 (10%)-NO (500 ppm)C3H6 (500 ppm)-N2 [100]

Fig. 5.7 Typical chromatogram plot of a gas phase from pulsed DBD processing O2 (8%)-NO (300 ppm)-C3H6 (150 ppm)-N2 (balance): SIE ¼ 36 J/L, and T ¼ 150
C [101]

From these studies, the reaction products from these steps form hydrocarbon intermediates that react with oxygen to form HO2 radicals that become the main component in the NO removal in the presence of C3H6 and C2H4. However, C3H6 is more beneficial compared with C2H4 for the cleaning process because of an additional formation of HC radicals. For a given specific input energy, the gas temperature affects the efficiency of oxidation of NO to NO2. Penetrante et al. [1] show that in gas mixtures without hydrocarbons, the efficiency of oxidation of NO to NO2 by O radicals drops as the

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Fig. 5.8 NO and NO2 concentrations as a function of gas temperature. Plasma processing of C3H6 (660 ppm)-NO (500 ppm)-O2 (8% vol.)-N2 at SIE ¼ 36 J/L [101]

gas temperature increased. At high temperatures, the NO to NO2 oxidation reaction is counteracted by the reduction reaction as demonstrated by Djéga-Mariadassou et al. [101] in mixture with propene (Fig. 5.8).

5.5 5.5.1

NOx Removal by Nonthermal Plasma and Energy Cost Effect of Hydrocarbons

The NO to NO2 conversion significantly depends on gaseous mixtures. As shown in the previous section, the hydrocarbon addition leads to significantly influence the NOx chemistry during plasma remediation leading to the promotion of the oxidation of NO to NO2 and lower the energy cost of this oxidation. According to the calculations of Park et al. [92], the initial HC concentration more than 500 ppm (except CH4), and O2 content more than 10%, almost does not affect the NO oxidation to NO2. Experimental work of Khacef et al. [22] on the NOx remediation in the presence of propene resulted in the similar conclusion as shown in Fig. 5.9. Figure 5.10 shows the NO and NOx (NO + NO2) conversion rate as a function of SIE for the cases without propene and with 500 ppm propene. In the absence of hydrocarbons, less than 55% of the initial NO is converted to NO2 even for the highest SIE, and the NOx removal was zero. When 500 ppm of propene is added to the gas stream, the oxidation of NO to NO2 was enhanced due to peroxy radicals HO2 (see previous section).

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Fig. 5.9 Effect of inlet propene concentration on NO to NO2 conversion in plasma processing of 500 ppm NO in air (T ¼ 100
C, SIE ¼ 44 J/L) [22]

Fig. 5.10 Effect of SIE on NO and NOx conversion without and with 500 ppm propene in NO (500 ppm)-air (195 mJ/pulse and repetition rate up to 40 Hz) [22]

5.5.2

Energy Cost

One of the relevant parameters in the cleaning of pollutant gas (NOx, VOCs, etc.) is the energy cost for removing the unwanted molecules. To be competitive with other technologies, the nonthermal plasma technology applied for cleaning industrial or automotive exhaust gases should be the less consuming energy. The energy cost of the plasma-chemical processes is closely related to its mechanisms, and the same plasma processes in different discharge systems or under different conditions result

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in entirely different expenses of energy. Usually, the plasma processing literature present the fraction of NOx removed as a function of parameters such as the specific input energy (SIE ¼ mean input power/flow rate), the residence time of the gas into the reactor, or the applied voltage to the plasma reactor. SIE is the parameter commonly used to evaluate the NOx conversion efficiency in the plasma. However, if this parameter is important to characterize the electrical energy consumption of the process, one should take into account the way to achieve the chosen energy value. It means that for a given plasma reactor, the plasma chemistry strongly depends on the energy transfer from the power source to plasma reactor. Many parameters, such as type of discharge (pulse, AC), high-voltage characteristics (amplitude, rise time, duration, frequency), electrode configuration, and efficiency of chemical reactions, have to be taken into account to optimize the process for a better energy efficiency. The lowering of energy cost for NOx conversion was obtained by many research groups for different systems. Niessen et al. [88] examine the effect of ethylene on the conversion of NOx in a DBD. They reported an improvement of NO removal efficiency by an order of magnitude in the presence of 2000 ppm C2H4. Without C2H4 in the gas mixture, 48 eV/molecule (experiment) and 61 eV/molecule (model) are needed for the removal of one NO molecule at 90% removal rate, while adding 2000 ppm C2H4 leads to energetic cost of only 6 eV/molecule (experiment) and 9.6 eV/molecule (model). In pulsed DBD experiments with gas mixture simulated diesel exhaust, Khacef et al. [22, 102] obtained a change of the energy cost for NO removal with temperature in the presence of C3H6. The authors demonstrate that, for a given reactor under the same gas composition and equivalent SIE, the NO removal efficiency is optimum for a low input energy per pulse and a high discharge frequency as shown in Fig. 5.11. The modeling of Filimonova and Amirov [103] on the removal of toxic impurities from air flow by a pulsed corona discharge resulted in similar conclusion. These authors confirm that it is possible to increase the removal efficiency at a constant total energy deposition by choosing the optimum pulse repetition rate and the energy deposited per discharge pulse. As example, for a SIE of about 27 J/L, NO conversion is more than 90% for 35 mJ/pulse and 200 Hz and less than 30% for 195 mJ/pulse and 30 Hz. Whatever the pulsed discharge operations used, the main end products of the process are NO2, CO, CO2, CH2O, CH3ONO2, CH2O2, HONO, and HONO2. However, marked differences in final concentrations of these species were observed according the operating conditions. Figure 5.12 shows an example of NOx concentrations measured at the exit of a pulsed DBD reactor as a function of SIE in dry and humid (10% H2O) gas mixture at 260
C. With increasing energy deposition, the NO conversion and NO2 formation efficiencies have been improved by increasing the gas temperature (up to 260
C) and by adding 10% H2O in the dry gas mixture. As example, the energy deposition of only 7 J/L was required to achieve maximum NO to NO2 oxidation efficiency (up to 92%). This efficiency rate was only 61% when no water is present in the gas stream.

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Fig. 5.11 NO and NO2 concentrations as a function of SIE for pulse energy of 35 and 195 mJ. Gas mixture: O2 (10%)-NO (500 ppm)-C3H6 (500 ppm)-N2 (balance) [102]

Fig. 5.12 Effect of SIE on NOx conversion at 260
C. Gas mixture: O2 (10%)-NO (500 ppm)C3H6 (500 ppm)-N2 (balance) without H2O and with 10% of H2O [22]

The energy cost for humid plasma-induced NO removal is dramatically reduced to values scaling from  15 eV/molecule at 27 J/L down to  4 eV/molecule at 7 J/L. Bröer et al. [104] investigated synthetic gas mixture containing higher O2 concentration (18%) with C2H4 instead of C3H6 and found that the maximum NO removal rate was 92% with an energy cost around 100 eV/molecule. In the modeling of the NO oxidation process in simulated exhaust containing O2-H2O-CO2-CO-NOH2-C3H6-C3H8-N2, Dorai and Kushner [90] observed that at 58 J/L, the energy cost decreases from 240 eV/molecule for a single pulse to 185 eV/molecule when the same energy was distributed over 20 pulses.

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For vehicle exhaust systems, it was established that the observed chemistry in the plasma includes the conversion of NO to NO2 as well as the partial oxidation of hydrocarbons. The presence of the unburned hydrocarbons (UHCs) in the exhaust is very important for the plasma- catalytic deNOx process for essentially four reasons [1, 2, 22, 50, 87–89, 95]. (a) UHCs enhance the gas-phase oxidation of NO to NO2 and lower the energy cost of this oxidation. (b) The partial oxidation of UHCs leads to produce chemical species such as aldehydes and alcohols useful for the catalytic reduction of NOx. For some catalysts, the partially oxygenated hydrocarbons are much more effective compared to original hydrocarbons in reducing NOx to N2. (c) The UHCs minimize the formation of acid products. (d) UHCs prevent the oxidation of SO2 thus making the plasma-catalytic process tolerant to the sulfur content of the fuel.

5.6 5.6.1

Plasma-Catalytic Process for NOx Abatement Why Do We Need Plasma for NOx Abatement in Industrial Processes

As already discussed in the literature, NOx trap systems and assisted NOx reduction using SCR catalysts are commonly the two industrial processes developed in order to reach the limitation in terms of NOx emission [5–8]. An alternative solution, not industrialized yet, is the use of hydrocarbons available in the exhaust, as reductant, to reduce the NOx. However, in order to reach low temperature NOx abatement, no catalytic system exists nowadays; thus, the combination of plasma and catalytic system would be a solution.

5.6.2

New Processes and Industrial Technologies

Atmospheric pressure NTP hybrid exhaust aftertreatment systems in the absence of noble metals has been developed by Kuwahara et al. [105]. Two types of new environmental protection systems (a dry system and a wet system), which enable the production of ultralow CO2, particulate matter, and NOx emissions as well as reduced fuel consumption and low cost, are investigated for diesel engines, marine engines, and combustion boiler applications. This paper reports the principles of the dry system and some recent experimental results of laboratory tests. The NOx reduction comprises three flow processes: (i) adsorption, (ii) heating, and (iii) cooling processes. The heating process corresponds to the regeneration process. This dry system demonstrates excellent energy efficiencies that meet Japanese

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national regulations regarding automobile diesel engine exhaust gas. In this study, approximately 60% of NOx conversion in the exhaust was achieved after 35 h on stream. An energy efficiency of about 143 g (NO2)/kWh was then achieved for NOx reduction. Recently, two types of innovative NTP environmental protection systems [106, 107] using non-noble metals and which enable the production of ultralow CO2, particulate matter (PM), and NOx emissions as well as reduced fuel consumption and cost were proposed. These NTP systems are realized for diesel hybrid engines, marine engines, and combustion boiler applications. In these systems, the plasma technology is combined with other environmental technologies such as adsorption or wet-type chemical scrubbers. These technologies are expected to be new global environmental protection technologies, as they will enable an avoidance of the difficulties in catalytic methods and ordinary plasma methods. Other systems using a combination of NTP process and normal EGR (exhaust gas recirculation) led to similar results [108, 109].

5.6.3

Lab-Scale and Fundamental Studies

5.6.3.1

SCR of NOx by Ammonia Assisted by Plasma

Only few studies dealt with selective catalytic reduction of NOx by ammonia assisted by plasma. Thus, the SCR of nitrogen oxides by a combination of DBD plasma and a monolithic V2O5-WO3/TiO2 catalyst using ammonia between 100 and 250
C was proposed [30]. Figure 5.13 summarizes the results obtained in that study. The gas mixtures used were representative of diesel exhaust gases. For gas mixtures in which 95% of the nitrogen oxides were in the form of NO, the removal of NOx below 150
C in the absence of plasma was negligible. Using the catalytic bed in the postdischarge DBD, about 70% of the NOx was reduced at temperatures as low as 100
C. Due to the coexistence of NO and NO2 on the catalyst, the selective catalytic reduction was enhanced. Similar effects were observed for the SCR in gas mixtures containing equal amounts of NO and NO2 without plasma treatment. This study clearly shows the efficiency of the plasma catalyst system at temperatures lower than those used for the catalytic system alone. The potential of plasma-enhanced SCR for NOx removal from exhaust gases containing high concentration of oxygen was investigated in a combination of dielectric barrier discharges with a monolithic V2O5-WO3/TiO2 catalyst using ammonia as a reducing agent. Experiments were performed which showed the temperature dependence of NOx removal from synthetic gas mixtures, for very low temperatures (between 100 and 250
C). It was shown that for T < 200
C, the NO reduction rate decreased sharply as long as NO2 concentration was low compared to NO concentration. This was the case when no plasma treatment was performed prior to catalytic treatment. With plasma treatment even at temperatures

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Fig. 5.13 DBD plasma-enhanced NH3-SCR of NOx (V2O5-WO3/TiO2 catalyst, synthetic gas mixture O2 (13%)-H2O (5%)-N2 (82%)-NOx (500 ppm)-NH3 (500 ppm) [30]

as low as 100
C, NOx reduction reached values up to 70%. An explanation of these results could be given by experiments with gas mixtures containing NO2 concentration, which were nearly equal to the NO concentration: without plasma treatment at 100
C, the degree of catalytic NOx reduction was 70%, also. From this result, it is concluded that on a V2O5-WO3/TiO2 catalyst at low temperatures, NO can be reduced in the presence of NO2. Similar kinetic behavior was found in reactions of NOx with zeolite-fixed ammonium ions [110]. In summary, one can conclude that the pre-treatment of the exhaust gas by nonthermal plasma enhances selective catalytic reduction rates at temperatures below 200
C by oxidation of NO to NO2, which was already proposed by McLarnon and Penetrante [75].

5.6.3.2

SCR of NOx by Hydrocarbons Assisted by Plasma

Published literature reports on the application of the plasma-assisted HC/SCR to the lean-NOx catalysis have focused primarily on the in situ activation by nonthermal plasma of the gas-phase reactants such as NO and/or HCs leading to NO2 and oxygenated species [101, 111, 112]. In these studies, alumina- or zeolite-based catalysts were used coupled with NTP DBD in the wide temperature range (100–500
C). The results clearly show that in the presence of the plasma, the catalytic activity is shifted to low temperature with high conversion of NO, and industrial application was clearly shown [110].

5 Plasma-Catalytic Removal of NOx in Mobile and Stationary Sources

5.7 5.7.1

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Role and Beneficial Effect of Plasma Importance of NO2 in the Catalytic Reduction of NOx

Djéga-Mariadassou and co-workers showed that a three-function catalyst model for SCR NOx with hydrocarbons could be proposed in the absence of plasma [101, 112, 113]. That model is based on experimental evidence for each function, during temperature programmed surface reactions (TPSR), and has been verified during stationary experiments. A general scheme of the model is shown in Fig. 5.14. The first function F1 leads to the oxidation of NO to NO2. The second function F2 is the mild oxidation of hydrocarbons through organic nitrogen-containing intermediates (R-NOx) leading to CxHyOz intermediate species such as aldehydes or alcohols. The third function F3 involves the NO reduction by subsequent formation of N2 assisted by the oxidation of reductants to CO2/H2O over transition metal cations. The previous CxHyOz intermediate species can achieve their own total oxidation by cleaning the adsorbed oxygen species left by NO dissociation. It is very difficult to find the best design of the catalyst to simultaneously initiate the three functions by itself. Thus, an external device can be developed to substitute functions F1 and F2, providing the catalyst the appropriate oxygenated species, in the full range of temperature. For the vehicle exhaust systems, it was established that the plasma chemistry includes the conversion of NO to NO2 as well as the partial oxidation of hydrocarbons. The presence of the unburned hydrocarbons in the exhaust is very important for the plasma-catalytic deNOx process for multiple reasons. First, unburned hydrocarbons enhance the gas-phase oxidation of NO to NO2 and lower the energy cost for this oxidation. Secondly, their partial oxidation leads to produce, in the whole range of reaction temperature, chemical species such as aldehydes, alcohols, and R-NOx useful for the catalytic reduction of NOx. For some catalysts, the partially oxygenated hydrocarbons are much more effective compared to original hydrocarbons in reducing NOx to N2. Thirdly, unburned hydrocarbons prevent the oxidation of SO2 thus making the plasma- catalytic process tolerant to the sulfur content of the fuel. These “intermediate” species are needed for function F1 and F2 and, furthermore, for the third function itself. The NTP could substitute for the first two functions (F1, F2) of the catalysts as already proposed elsewhere [30, 101, 105–109, 112] (Fig. 5.15).

Fig. 5.14 Scheme for the three-function catalyst: concerted actions for the selective reduction of NO by hydrocarbons (CxHyOz, partially oxidized HC)

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Fig. 5.15 Three-function mechanism adapted for plasma-catalytic processes

This result demonstrates what can be the role of plasma in nonthermal plasmaassisted catalytic NOx remediation. Furthermore, plasma is able to provide both NO2 and CxHyOz intermediate species at low temperature. However, the behavior of the R-NOx species formed in the plasma still remains to be studied. The NOx removal leads mainly to N2 as reported previously [114].

5.7.2

On the Effect of the Catalysts in the Coupled Process

Miessner et al. [34] have shown that the combination of HC/HCR and nonthermal plasma enhances the overall reaction and allows an effective removal of NOx at relatively low temperature (Fig. 5.16). The oxidative potential of an NTP in off-gases with excess oxygen results in an effective conversion of NO to NO2 that can be converted synergistically by HC/HCR to molecular nitrogen with appropriate catalysts. The hydrocarbon added has two essential functions: first, it assists the gas-phase oxidation of NO to NO2 by the electric discharge in excess oxygen and, secondly, it reacts with NO2 in the hydrocarbon SCR. Besides CO2 and CO, significant amounts of formaldehyde and acetaldehyde are formed in the plasmainitiated gas-phase reaction. These and other by-products are involved together with the remaining propene in the subsequent catalytic reaction. With a specific input energy density lower than 15 Wh/m3, a temperature of 300
C, and a space velocity of 20,000 h1, NOx conversions higher than 50% are obtained. The synergistic combination of NTPs and HC/HCR has been verified under real conditions with exhaust gas from a diesel engine. The catalyst support itself, Al2O3, seems to be an appropriate catalyst for the reaction [34, 114, 115], and some modifications of Al2O3 and ZrO2 were found to be effective as catalysts in this reaction. The role of plasma processing on NOx reduction over alumina and basic zeolite NaY was examined by Cho et al. [111] and compared to a conventional NH3/SCR system, and the results are comparable at low temperature (Fig. 5.17).

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Fig. 5.16 Efficiency of various catalysts in the plasma-catalytic HC/HCR of NOx (500 ppm NO and 1000 ppm C3H6 in N2/O2 (13%); SIE ¼ 14 Wh/m) [34]

Fig. 5.17 Plasma-catalytic system and conventional NH3/SCR in engine dyno tests as a function of temperature [111]

During the plasma treatment, NO is oxidized to NO2, and propylene is partially oxidized to CO, CO2, acetaldehyde, and formaldehyde. With plasma treatment, NO as the NOx gas, and a NaY catalyst, the maximum NOx conversion was 70% between 180 and 230
C. The activity decreased at higher and lower temperatures. As high as 80%, NOx removal over alumina was measured. For both catalysts, a simultaneous decrease in NOx and aldehyde compound concentrations was observed, which suggests that aldehydes may be important components for NOx reduction in plasma-treated exhaust, which was already proposed by DjégaMariadassou et al. [101, 112, 113, 116]. The authors proposed a simplified reaction

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mechanism to explain the enhanced deNOx activity induced by air plasma as follows: ozone production, NO and HC oxidation by ozone, and NOx reduction assisted by oxidized hydrocarbons to form N2, H2O, and CO2. That later reaction proceeds on BaY catalyst at low temperatures and on CuCoY catalyst at high temperatures as evidenced by Schmieg et al. [115] and Lee et al. [117].

5.8

Conclusion

NTP catalytic processes are in development in order to reduce the NOx in the exhaust gases from industrial processes such as burners for stationary sources or in the exhaust line for mobile applications. It is clear that the plasma is able to create at low temperature the needed species for NOx abatement. Some coupled processes have been already tested at the pilot scale, but their industrialization will be possible only if the energy due to the plasma makes this process a low-cost process versus the existing ones. Another possible way to use the plasma to reduce NOx emission is the use of this one in the combustion process to avoid the NOx formation. Thus, Lee et al. [118] have recently presented a new combustion technology that can reduce NOx emissions within industrial burners to single-digit parts per million levels without employing exhaust gas recirculation or other NOx reduction mechanisms. This new technology uses a simple modification of commercial burners, such that they are able to perform plasma- assisted staged combustion without altering the outer configuration of the commercial reference burner. Because this plasma burner acted as a low NOx burner and was able to reduce NOx by more than half compared to the commercial reference burner, this methodology offers important cost-effective possibilities for NOx reduction in industrial applications at source. Acknowledgments The authors appreciate the considerable contribution of Professor Gérald Djéga-Mariadassou and Doctor Francois Baudin as well as engineers from Renault SA and PSA Peugeot-Citröen for their fruitful discussions on deNOx processes and engine chemistry.

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85. Fresnet, F., Baravian, G., Magne, L., Pasquiers, S., Postel, C., Puech, V., & Rousseau, A. (2002). Influence of water on NO removal by pulsed discharge in N2/H2O/NO mixtures. Plasma Sources Science and Technology, 11, 152–160. 86. Zhao, G. B., Hu, X., Yeung, M. C., Plumb, O. A., & Radosz, M. (2004). Non-thermal plasma reactions of dilute nitrogen oxide mixtures: NOx in nitrogen. Industrial and Engineering Chemistry Research, 43, 2315–2323. 87. Shin, H. H., & Yoon, W. S. (2000). Effect of hydrocarbons on the promotion of NO-NO2 conversion in non-thermal plasma deNOx treatment. SAE Tech Paper 2000-01-2969. 88. Niessen, W., Wolf, O., Schruft, R., & Neiger, M. (1998). The influence of ethene on the conversion of NOx in a dielectric barrier discharge. Journal of Physics D: Applied Physics, 31, 542–550. 89. Filimonova, E. A., Kim, Y. H., Hong, S. H., & Song, Y. H. (2002). Multi-parametric investigation on NOx removal from simulated diesel exhaust with hydrocarbons by pulsed corona discharge. Journal of Physics D: Applied Physics, 35, 2795–2807. 90. Dorai, R., & Kushner, M. J. (2001). Effect of multiple pulses on the plasma chemistry during the remediation of NOx using dielectric barrier discharges. Journal of Physics D: Applied Physics, 34, 574–583. 91. Tsang, W. (1991). Chemical kinetic data base for combustion chemistry Part V. Propene. Journal of Physical and Chemical Reference Data, 20, 221–274. 92. Park, K. S., Kim, D. I., Lee, H. S., Chun, K. M., & Chun, B. H. (2001). Effect of various hydrocarbons on the plasma deNOx process. SAE Tech Paper 2001-01-3515. 93. Wilk, R. D., Cernansky, N. P., Pitz, W. J., & Westbrook, C. K. (1989). Propene oxidation at low and intermediate temperatures: A detailed chemical kinetic study. Combustion Flame, 77, 145–170. 94. Martin, A. R., Shawcross, J. T., & Whitehead, J. C. (2004). Modelling of non-thermal plasma aftertreatment of exhaust gas streams. Journal of Physics D: Applied Physics, 37, 42–49. 95. Dorai, R., & Kushner, M. J. (1999). Effect of propene on the remediation of NOx from engine exhausts. SAE Tech Paper 1999-01-3683. 96. Dorai, R., & Kushner, M. J. (2000). Consequences of propene and propane on plasma remediation of NOx. Journal of Applied Physics, 88, 3739–3747. 97. Atkinson, R. (1997). Gas-phase tropospheric chemistry of volatile organic compounds: 1. Alkanes and alkenes. Journal of Physical and Chemical Reference Data, 26, 215–290. 98. Hoard, J. W., & Panov, A. (2001). Products and intermediates in plasma-catalyst treatment of simulated diesel exhaust. SAE Tech Paper 2001-01-3512. 99. Koda, S., Endo, Y., Tsuchiya, S., & Hirota, E. (1991). Branching ratios in atomic oxygen (3P) reactions of terminal olefins studied by kinetic microwave absorption spectroscopy. The Journal of Physical Chemistry, 95, 1241–1244. 100. Khacef, A., Cormier, J. M., & Pouvesle, J. M. (2005). Non-thermal plasma NOx remediation: From binary gas mixture to lean-burn gasoline and diesel engine exhaust. Journal of Advanced Oxidation Technologies, 8, 150–157. 101. Djéga-Mariadassou, G., Baudin, F., Khacef, A., & Da Costa, P. (2012). NOx abatement by plasma catalysis. In V. I. Parvelescu, M. Magureanu, & P. Lukes (Eds.), Plasma chemistry and catalysis in gases and liquids (pp. 89–129). Weinheim: Wiley-VCH Verlag GmbH. 102. Khacef, A., Cormier, J. M., & Pouvesle, J. M. (2006). Energy deposition effect on the NOx remediation in oxidative media using atmospheric non-thermal plasmas. European Physical Journal Applied Physics, 33, 195–198. 103. Filimonova, E. A., & Amirov, R. K. (2001). Simulation of ethylene conversion initiated by a streamer corona in an air flow. Plasma Physics Reports, 27, 708–714. 104. Bröer, S., Hammer, T., & Kishimoto, T. (1997, September 8–12) NO-removal in hydrocarbon containing gas mixtures induced by dielectric barrier discharges. Paper presented at the 12th international conference of gas discharges and their applications, Greifswald, Germany, pp 188–192.

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105. Kuwahara, T., Yoshida, K., Kannaka, Y., Kuroki, T., & Okubo, M. (2011). Improvement of NOx reduction efficiency in diesel emission control using non-thermal plasma combined exhaust gas recirculation process. IEEE Transactions on Industry Applications, 47, 2359–2366. 106. Okubo, M. (2008). Air and water pollution control technologies using atmospheric pressure low temperature plasma hybrid processes. Journal of Plasma and Fusion Research, 84, 121–134. 107. Okubo, M., Arita, N., Kuroki, T., & Yamamoto, T. (2008). Total diesel emission control system using ozone injection and plasma desorption. Plasma Chemistry and Plasma Processing, 28, 173–187. 108. Yoshioka, Y., Takahashi, T., Togashi, T., & Shoyama, T. (2007). Efficient NO removal from diesel exhaust gases by a combination of ozone injection and exhaust gas recirculation. Journal of Advanced Oxidation Technologies, 10, 304–310. 109. Yoshioka, Y. (2007). Recent development in plasma De-NOx and PM (particular matter) removal technologies from diesel exhaust gases. International Journal of Plasma Environmental Science and Technology, 1, 110–122. 110. Richter, M., Eckelt, R., Parlitz, B., & Fricke, R. (1998). Low-temperature conversion of NOx to N2 by zeolite-fixed ammonium ions. Applied Catalysis B: Environmental, 15, 129–146. 111. Cho, B. K., Lee, J. H., Crellin, C. C., Olson, K. L., Hilden, D. L., Kim, M. K., Kim, P. S., Heo, I., Oh, S. H., & Nam, I. S. (2012). Selective catalytic reduction of NOx by diesel fuel: Plasmaassisted HC/SCR system. Catalysis Today, 191, 20–24. 112. Gorce, O., Jurado, H., Thomas, C., Djéga-Mariadassou, G., Khacef, A., Cormier, J. M., Pouvesle, J. M., Blanchard, G., Calvo, S., & Lendresse, Y. (2001) Non-thermal plasma assisted catalytic NOx remediation from a lean model exhaust. SAE Tech Paper 2001-01-3508. 113. Djéga-Mariadassou, G., Berger, M., Gorce, O., Park, J. W., Pernot, H., Potvin, C., Thomas, C., & Da Costa, P. (2007). A three-function model reaction for designing deNOx catalysts. In P. Granger & V. I. Parvulescu (Eds.), Past and present in deNOx catalysis: From molecular modelling to chemical engineering (pp. 145–173). The Netherlands: Elsevier. 114. Baudin, F. (2005). PhD Thesis, Université Pierre et Marie Curie Paris, France. 115. Schmieg, S. J., Cho, B. K., & Oh, S. H. (2004). Selective catalytic reduction of nitric oxide with acetaldehyde over NaY zeolite catalyst in lean exhaust feed. Applied Catalysis B: Environmental, 49, 113–125. 116. Djéga-Mariadassou, G., Fajardie, F., Tempère, J. F., Manoli, J. M., Touret, O., & Blanchard, G. (2000). A general model for both three-way and deNOx catalysis: Dissociative or associative nitric oxide adsorption, and its assisted decomposition in the presence of a reductant: Part I. Nitric oxide decomposition assisted by CO over reduced or oxidized rhodium species supported on ceria. Journal of Molecular Catalysis A: Chemical, 161, 179–189. 117. Kim, M. K., Kim, P. S., Nam, I. S., Cho, B. K., & Oh, S. H. (2012). Enhanced NOx reduction and byproduct removal by (HC+OHC)/SCR over multifunctional dual-bed monolith catalyst. Catalysis Today, 184, 95–106. 118. Lee, D. H., Kim, K. T., Kang, H. S., Song, Y. H., & Park, J. E. (2013). Plasma assisted combustion technology for NOx reduction. Environmental Science and Technology, 47, 10964–10970.

Chapter 6

Plasma-Catalytic Removal of VOCs Pieter Cools, Nathalie De Geyter, and Rino Morent

6.1 6.1.1

Introduction Relevance

The aim of this chapter is to discuss the abatement of volatile organic compounds (VOCs) using nonthermal plasma (NTP) in combination with a catalyst. The first experiments, using this technique, were done in the 1950s by Dennis and Burton [1]. These, along with other pioneers like Gicquel et al., laid the foundation for plasma catalysis as it is known today [2–4]. Since the Kyoto protocol in 1997, there has been a growing public awareness and concern around environmental pollution. This has stimulated research groups around the world to further explore the possibilities of NTP catalysis as an eco-friendly technique for the abatement of dilute VOCs present in waste gas streams and indoor air. NTP catalysis is an attractive technique for controlling air pollution for a number of reasons. It is a quick start-up process; can be operated in ambient conditions, thus avoiding the use of extensive vacuum equipment; and reacts quickly to changes in waste gas composition [5]. NTP without the assistance of a catalyst used to be the more popular technique for VOC abatement, but a review of the papers collected through Web of Science shows that since 2004, plasma catalysis has become the more popular technique, with a steady 15–20 publications a year (Fig. 6.1). In the last 2–3 years, notably more papers have been published, which seems to point toward a positive trend.

P. Cools · N. De Geyter · R. Morent (*) Research Unit Plasma Technology, Department of Applied Physics, Ghent University, Sint-Pietersnieuwstraat, Ghent, Belgium e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2019 X. Tu et al. (eds.), Plasma Catalysis, Springer Series on Atomic, Optical, and Plasma Physics 106, https://doi.org/10.1007/978-3-030-05189-1_6

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Fig. 6.1 Number of publications on plasma catalysis since 1996

6.1.2

What Is Plasma Catalysis?

The use of NTP as such for the abatement of VOCs, SOx, NOx, and odors was, and still is, an attractive technique, but there is a general consensus in the scientific community that there are some drawbacks inherently linked with this approach, i.e., a poor energy efficiency, a low degree of mineralization, and incomplete destruction and oxidation resulting in the production of harmful by-products such as NOx, CO, and new VOCs. Two approaches are available for combining NTP with a heterogeneous catalyst: in-plasma catalysis (IPC) and post-plasma catalysis (PPC). The first is a single-step process putting the catalyst in the active plasma zone, while the latter consists of a two-stage process, positioning the catalysts downstream of the active plasma region (see Fig. 6.2). The used nomenclature in literature for IPC and PPC is not unilaterally spread and might cause confusion for readers not experienced in the field. For PPC, one can find terms such as post-plasma catalysis reactor (PPCR) [6], plasmaenhanced catalysis (PEC) [7], or two-stage plasma catalysis (TSPC) [8]. Synonyms of IPC are combined plasma catalysis (CPC) [9–11], in-plasma catalysis reactor [6], plasma-driven catalysis [7], plasma and catalyst integrated technologies (PACT) [12], and single-stage plasma catalysis [8]. Plasma catalysis is generally considered to be a synergetic process, as in some cases the plasma is responsible for the (re) activation of the heterogeneous catalyst. UV, local heating, O3, activation of lattice oxygen, adsorption/desorption, changes in work function, direct interaction of gas-phase radicals with adsorbed pollutants, and the creation of electron-hole pairs are considered to be the most prominent activation mechanisms [13].

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Fig. 6.2 Different configurations for plasma catalysis systems [15]. (a) Single-stage plasma catalysis reactor. (b) Two-stage plasma catalysis reactor. (c) Multi-stage plasma catalysis reactor

The interactions between catalyst and plasma described in the next few paragraphs can all be directly linked to one or more of the activations mechanisms mentioned above. The current experimental results and modeling data are originating from specific working conditions [14]. As such, the collection of data available might appear to be no more than scattered pieces of information. It is therefore indeed correct to assume that a continued effort is required from the plasma community to further unravel the mechanisms leading to those results and fill in the remaining blanks. However, at this stage, it is still of great value to collect and organize an overview on the general trends, dependencies, and limitations for NTP catalysis.

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6.1.3

Influence of the Catalyst on the Plasma Processes

6.1.3.1

Discharge Mode

The introduction of a heterogeneous catalyst into the discharge zone will affect the physical properties of the discharge. A simple example with complicated consequences is the introduction of a dielectric surface in between the gap of a filamentary discharge. Whereas the discharge mode was originally characterized by bulk streamers, it now converts to a surface flashover system which will be characterized by more intense streamers running along the surface [16]. By changing the catalyst pellet size, it is possible to further distort the electrical field in the voids between the pellets, as the dielectric constant will affect the electric field generated in the voids between the pellets and therefore alter the mean electron energy [17]. By decreasing the pellet size, the number of micro-discharges will increase progressively, but the charge intensity that is transferred per micro-discharge will decrease. The addition of ferroelectric pellets into the discharge is another common practice which, under normal conditions, leads to higher average electron energies, resulting in a more oxidative discharge [18].

6.1.3.2

Reactive Species Production

Besides changing the physical characteristics of the plasma, the introduction of a heterogeneous catalyst also affects the chemical activity of the system. The oxidation of various organic compounds immobilized on (non)-porous silica and alumina catalysts was studied by Roland et al. [19]. The overall conclusion was that the several short-living species are generated in the pore volume of the porous materials when exposed to the plasma. Then again, the opposite effect was observed by other researchers, the catalyst being responsible for a reduction in ionic species [20]. This said, in this case, the reduction of ionic species did not inhibit the catalyst’s efficiency in reducing the emission of CO and O3.

6.1.4

Influence of the Plasma on the Catalytic Processes

6.1.4.1

Catalyst Properties

It is possible to use NTPs for catalyst preparation [21–26]. Plasma activation of the catalyst surface improves the catalytic activity and stimulates the dispersion of the catalytic components [27–29]. NTPs are furthermore capable of changing the oxidative state of the catalyst. Mn2O3, a commonly used catalyst, can convert into Mn3O4 upon prolonged exposure to a DBD plasma [29]. This lower-valent manganese oxide is known for its higher oxidation potential. For titanium dioxide, less Ti-O bonds are registered after long-term exposure to a plasma discharge [30]. Also,

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more uncommon metastables can be formed on the catalyst surface. Al-O-O
is the best known example of a long-term metastable found within the pores of an aluminum oxide catalyst up to 2 weeks after the IPC run [30]. Finally, plasma exposure is also known to actively change the specific surface area and/or structure of the exposed catalyst [28, 29, 31].

6.1.4.2

Thermal Activation

The heat generated by the plasma during IPC will result in a higher catalyst surface temperature but is as such insufficient to cause thermal activation of the catalyst. However, due to intense micro-discharges running between sharp edges and the pellets, local hot spots can occur in packed bed reactors [32], at those localized points, catalyst temperatures can reach sufficiently high levels to enhance the catalytic breakdown of VOCs [33].

6.1.4.3

Adsorption

The role adsorption processes play during IPC cannot be neglected and might considerably alter the reaction mechanisms. Depending on the adsorption capacity of the catalyst, pollutant retention time can be increased (good adsorption capacity) or reduced (low adsorption capacity). The use of a more porous catalyst normally results in a prolonged residence time of both the VOC and the generated plasma species [34]. If the VOC residence within the plasma is prolonged, the probability of collision with an active species within the plasma increases, enhancing the overall VOC removal efficiency. Besides the adsorption of pollutants, other compounds such as water and ozone might compete for the active sites on the catalyst surface. Humidity is an essential component for both IPC and PPC processes and often causes a reduction in VOC conversion due to a decrease in reaction probability between the catalyst surface and the pollutant [35]. The adsorption of ozone on certain catalysts results in its dissociation into highly reactive atomic oxygen, a process that is considered favorable if the plasma-generated ozone in itself is not able to break down the VOC [36]. Besides sequential adsorption/activation, the cyclic operation of VOC adsorption and NTP-assisted regeneration has received considerable interest in the last few years, as first proposed by Ogata et al. [37, 38]. As discussed in the review paper by Sultana et al., the use of a cyclic adsorption/activation scheme can lead to a more optimized energy consumption and a better cost effectiveness [39]. Furthermore, under certain conditions, it is found to lead to a higher CO2 selectivity and a reduced production of ozone. Several papers will be discussed further on in the chapter that makes use of cyclic adsorption, which can be recognized by their improved energy density [40–44].

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Plasma-Mediated Activation of Photocatalysts

Photocatalysis is a process where upon VOC adsorption, the semiconducting catalyst is exposed to a UV source. The UV radiation then generates electron-hole pairs within the semiconducting material, inducing oxidation of the adsorbed pollutant through valence band holes. In a final step, the newly generated oxidized end-products are then resorbed within the gas stream. TiO2 is by far the most popular photocatalyst, as it is most suitable for the processing of a wide range of VOCs. When combined with an NTP, the conversion efficiency of TiO2 substantially increases, and a higher selectivity toward CO2 can be achieved. Although it was initially believed that the UV light generated by the NTP was responsible for the photocatalytic effect, later research has shown that the wavelength shorter than 388 nm is needed to form up an electron-hole pair, as the bandgap for anatase phase TiO2 is 3.2 eV. Those few excited nitrogen states that do emit radiation beneath the maximum wavelength are not sufficient to explain the synergetic effect experimentally observed by several research groups. Sano et al. performed a series of experiments where the walls of their reactor were coated with a TiO2 layer to analyze the changes in acetylene conversion efficiency [45]. The UV light emitted by the plasma was adsorbed by the photocatalyst, but the intensity of the radiation was considered too low to initiate photoactivation. Similar experiments performed by other research groups confirmed these experimental findings [46–50]. Lee et al., among others, ran a series of experiments, using plasma-assisted photocatalysis for the abatement of benzene [48]. Besides the addition of O2, the used dilution gas was changed from N2 to Ar, resulting in considerably higher abatement efficiencies. These data strongly indicate that the contribution of the plasma generated UV radiation is negligible, as the emission range for excited Ar lies somewhere between 400 and 850 nm. Even though, there are some groups that assigned some of the photocatalytic activation to the UV light alone or in combination with the (re) activation of the catalyst emitted by the plasma [51]. The exact mechanisms driving the plasma-assisted photocatalysis are not known to date. Different theories on how the TiO2 band gap can be bridged could be formulated, but unfortunately, there is still insufficient information available on the relative importance that ions, electrons, metastables, surface recombination effects, and charging play within the complex mix that forms up NTP.

6.1.5

Different Types of Catalysts

Both for IPC and PPC systems, the heterogeneous catalyst can be introduced in a number of ways. The most common practice is the introduction of pellets (packed bed reactor), but also the use of foams and honeycomb monoliths has been studied [18, 20, 28, 29, 35, 52–61]. Besides filling up the reactor system, it is also possible to

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use a single layer of catalyst material, coat the reactor walls, or use the electrodes themselves as catalyst material [12, 51, 62–68]. Many different catalysts have been used in IPC and PPC systems for the abatements of VOCs. Porous adsorbents placed within the plasma were the first materials tested, as it was the common belief that the increase in retention time for the pollutants would give cause to a higher interaction probability between the active plasma components and said pollutants [37, 69]. Common examples of such materials were Al2O3, molecular sieves, and zeolites [19, 37, 38, 70–75]. Metals such as silver, palladium, nickel, copper, manganese, rhodium, cobalt, platinum, and manganese were incorporated into the porous catalyst support structure to provide for the catalytic activity [11, 13, 31, 33, 36, 72, 76–91]. In more recent years, TiO2 has been combined with a number of different supports and adsorbents and enriched with metals and metal oxides [13, 36, 49, 50, 73, 92–96]. Common examples of supports include beads, (glass) fibers, activated carbon, silica gel pellets, and nickel foam [47, 48, 56, 63, 97–102].

6.2

Critical Process Parameters

Before giving an in-depth discussion on the abatement of the most common VOCs by plasma catalysis, a number of critical process parameters will be listed that define the initial condition of the gas waste stream fed into the plasma catalysis system. For each parameter, the impact on the abatement efficiency will be reviewed and compared if possible.

6.2.1

Humidity Level

Humidity is an essential parameter within the plasma catalytic process. From an industrial point of view, it is highly relevant, as waste streams usually contain fluctuating concentrations of water vapor. The influence of the vapor on the abatement efficiency is highly dependent on the initial VOC concentration, the type of applied discharge, and the type of VOC targeted. In the plasma chemical process, water plays a prominent role as it decomposes into the highly active OH and H radicals according the to the following equations: H2 O þ e ! OH • þ H • þ e
H2 O þ N2 A3 Σþ u ! N2 þ OH • þ H •
H2 O þ O 1 D ! 2 OH •

ð6:1Þ ð6:2Þ ð6:3Þ

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Fig. 6.3 Negative correlation between toluene removal efficiency (TRE) as a function of relative humidity (left) and initial VOC concentration (right) [35, 136]

The oxidative capacity of the OH radical is considerably higher compared to other oxidative species such as atomic oxygen or peroxyl radicals. The influence of humidity has been mainly investigated on (packed-bed) DBD systems. For such systems, high humidity is known to negatively affect the total charge transfer within a micro-discharge, ultimately lowering the total volume of the active plasma region [103]. For corona discharges, similar findings have been reported [104]. The addition of water vapor to the waste stream reduces the currents observed for a given voltage, which can be assigned to the higher probability of the plasma attachment process [35]. This in turn leads to a lowered OH radical generation. The electronegativity of the water molecules also negatively affects the VOC decomposition, as it lowers the electron density and quenches other reactive chemical species formed within the active plasma region, as shown in Fig. 6.3, left. For a negative corona discharge, Ge et al. found that the optimal relative humidity was between 40 and 60% at room temperature [105]. Independent of the selected VOC, a negative correlation has been found between the presence of water vapor and the characteristics of the plasma discharge. Not all is negative though, as the addition of water does improve the generation of OH radicals for certain VOCs, which is a process directly in competition with the abovementioned observations [106]. Depending on the target VOC, these competing processes either can result in a suppression [102, 107–121] and an enhancement [100, 111, 122–130] or have no effect at all on the abatement efficiency [75, 111, 114, 131–137]. There are several papers available describing the possibility of having an optimal relative humidity for the abatement of a target VOC. For both toluene and TCE, this lies around 20% [35, 104, 138]. Additionally, water is known to compete for the consumption of atomic oxygen, thus lowering the production of O3 during plasma catalysis [139]. Finally, several papers also describe the positive influence of higher relative humidity on the selectivity toward CO2 generation at the expense of CO [99, 112, 133, 140]. For systems set up in a post-plasma catalysis configuration, catalytic ozonation will be of minor importance, as the formation of O3 will be prevented by the water vapor present. Furthermore, the water vapor directly competes for the active

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adsorption sited of the catalyst with the O3 molecules and the target VOC, thus lowering the probability of direct VOC/catalyst interaction [36, 57, 104, 141]. This kept in mind, it is of the uttermost importance to select catalysts with those chemical compositions and morphologies that are not greatly influenced by the adsorption of water. Besides competing for the active sites, increased concentrations of water vapor can effectively poison the catalyst, thus progressively lowering the catalyst performance [57, 141].

6.2.2

Initial VOC Concentration

The study on the variation of the initial VOC concentration is of great importance, as the VOC concentration in waste streams generated during industrial processes can strongly vary. A higher initial VOC concentration usually means a decrease in the electron density per VOC molecule as well as a reduction of the other active plasma species per VOC molecule. A general consensus is found in literature that a too high initial pollutant concentration is detrimental for the abatement efficiency in plasmacatalytic systems (see Fig. 6.3, right). In literature, several papers can be found that make the correlation between an increase in the characteristic energy (energy needed to abate 63% of the initially fed VOC concentration) and the energy yield as a progressive function of the pollutant concentration [114, 136, 142–147]. For certain halogenated hydrocarbons, the starting concentration only marginally influences the decomposition efficiency [132, 148–152]. This alternative behavior can be explained by a secondary decomposition step, the result of fragment ion and radical interactions, species that were produced in the primary decomposition step [152, 153]. Others claim that for this class of molecules, the primary decomposition by the active plasma species forms the rate-determining step, thus resulting in analog abatement efficiencies regardless of the used starting concentration [148].

6.2.3

Temperature

For most plasma-catalytic processes, an increase in temperature usually results in more efficient pollutant decomposition. This trend can be attributed to a higher reaction rate between atomic oxygen/ hydroxyl radicals and the pollutant exposed to them, as the degradation process is usually considered to be endothermic [58, 130, 154–161]. For those processes where electron impact is the predominant form of pollutant decomposition (e.g., CCl4), rather than radical interactions, no temperature dependency is observed, because the electron density is only marginally dependent of temperature [162, 163]. Still, an increase in temperature can lead to an improved abatement efficiency, as the overall energy density will increase, leading to an improved destruction of the VOC [164].

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A second plausible explanation for the improved abatement at elevated temperatures is the increase of the reduced electric field (E/n), being the ratio of the electric field (E) and the gas density (n). The reduced electric field is an essential factor in determining the overall electron energy of the discharge. At a constant pressure, an increase in gas temperature usually leads to a reduction of the gas density; the generated discharge will therefore be generated at an improved reduced electrical field [130, 136]. For post-plasma catalysis systems, the ozone produced by the plasma discharge can be dissociated in the gas phase as described in Eq. 6.4. O3 þ O2 ! O þ O2 þ O2

ð6:4Þ

At higher temperatures, the reaction rate for the O3 will profoundly accelerate (e.g., from 100  C to 300  C, a five times increase is found). Even though, the lifetime of the atomic oxygen is too short-lived to interact with the adsorbed pollutants at the catalyst surface. Besides a net increase in ozone decomposition, the reaction rate between adsorbed oxygen and pollutant will also be significantly increased. As these two effects are in direct competition, it forms the most plausible explanation for the different influences of temperature found in literature: with a higher temperature setting, the VOC abatement can increase [83, 85, 161, 165], decrease [58], or be independent of temperature variation [72].

6.2.4

Oxygen Content

The addition of oxygen to the waste gas stream influences the character of the generated discharge and takes a crucial role in the plasma chemistry. Small increments of oxygen concentration have a pronounced impact on the production of atomic oxygen, one of the key components for efficient VOC abatement. The addition of oxygen can also negatively impact the process, as its electronegative character can lead to electron attachment reactions, thus limiting the overall electron density and altering the electron energy distribution functions [119, 166]. Furthermore, both oxygen and atomic oxygen are capable of reacting with nitrogen and its excited states, thus competing directly for reactive species otherwise available for VOC abatement [167–169]. Taking the abovementioned characteristics of oxygen into account, it comes as no surprise that it is possible to optimize the oxygen content for the abatement of target VOCs. For most pollutants, the ideal oxygen concentration range lies between 1% and 5%. However, for most industrial processes, it is cumbersome to keep the oxygen level constant, as the waste stream composition is highly dependent on the industrial environment, more often than not consisting of ambient air. For IPC, the direct interaction between VOCs adsorbed on the surface of the catalyst and the generated oxygen radicals leads to a more pronounced VOC abatement [170]. Furthermore, it is known from literature that the NOx production is inhibited upon the addition of moderate amounts of oxygen [171].

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Fig. 6.4 From left to right: structural representation of toluene, benzene, and TCE

6.2.5

Gas Flow Rate

In literature, the used gas flow rates typically vary between 0.1 and 10 L/min. Increasing the flow rate results in a reduced VOC residence time within the hybrid catalytic system. This means that the reaction probability between the target VOC and the plasma-generated active species will be reduced, resulting in a lowered VOC abatement efficiency. For the VOC adsorption probability on the catalyst surface, the same argumentation can be followed. With the industrial process in mind, some research groups have tested multistage nonthermal plasma reactors, trying to improve the VOC residence time at higher gas flow rates, although this rarely has a positive impact on the energy efficiency of the VOC abatement process [50, 165, 172–175].

6.3

VOC Abatement

In the following chapter paragraphs, a more detailed overview will be given on the most frequently studied VOCs, i.e., toluene, benzene, and trichloroethylene (TCE) (see Fig. 6.4).

6.3.1

Toluene

Toluene, a by-product of the production of gasoline, is a commonly used industrial solvent found in paint thinners and glues and as a precursor for the synthesis of benzene. Compared to other chemicals typically studied in the field of plasma catalysis, it is not a carcinogenic, but exposure to low levels of the compound can lead to confusion, memory loss, hearing loss, nausea, and loss of color vision. Li et al. used a DC corona discharge in combination with a titanium dioxide photocatalyst [176]. The placement of the catalyst between the needle and mesh electrodes resulted in a more intense plasma discharge caused by a higher streamer repetition rate. This system configuration performed at its best giving an energy efficiency of 7.2 g/kWh and was able to reach 76% abatement. Without the presence of the photocatalyst, both the abatement efficiency (44%) and energy efficiency (3.2 g/kWh) are significantly lower. The increase of both parameters upon addition of the catalyst is attributed to the simultaneous oxidation of toluene in the waste stream and adsorbed on the catalyst surface and the activation of the titanium dioxide

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by the active plasma species. By operating the system in an intermittent regime, Li et al. claim that the efficiency can be further improved, as the catalyst surface is able to regenerate through surface desorption during the discharge. A macro-porous γ–Al2O3 catalyst packing within a DBD system was compared to a packing of non-adsorbing glass beads by Song et al., and they observed a higher absorbance of toluene at 100  C [70]. The production of ozone and nitric acid by-products was reduced as well. A paper published on a similar topic by Malik et al. showed that a higher overall surface area of the alumina packing allowed for a reduction of ozone production without influencing the overall abatement efficiency [177]. In 2007, Guo et al. performed a systematic study on IPC, using different catalysts [178]. Among those tested, a MnOx/Al2O3/Ni foam used in an IPC configuration came out as being the most effective [29]. Compared to a plasma alone system, the manganese oxide catalyst significantly enhances the energy yield. Based on an OH-rad detection method developed by the group, they were able to conclude that the improved toluene abatement could be primarily contributed to the efficient reaction process between the OH
and the toluene adsorbed on the activated catalyst as well as with the other activated species present on the catalyst. An increased humidity resulted in the coverage of the catalysts’ active sites, lowering the probability of reaction between catalyst and pollutant [179]. These experimental findings were confirmed by both Van Dormer et al. and Huang et al., as they both showed that water adsorbs onto the catalyst clustering into mono- and/or multilayer systems that protect the active sites from the waste stream [56, 57, 180, 181]. For the toluene to interact with an active site, it now has to diffuse through the watery layer. In the 2011 paper by Huang et al., a wire-plate DBD IPC system is filled with the aforementioned manganese oxide catalyst [182]. A systematic investigation was performed on the effect of humidity on the overall abatement efficiency of toluene, the generated CO2 selectivity, carbon balance, and O3 concentration. The addition of progressive amounts of water to the waste stream leads to an increasing reduction in O3 formation, as the high-energetic electrons are quenched by the H2O. Furthermore, there is active competition between the adsorption of water and O3 on the catalyst’s active sites. Optimized CO2 selectivity and carbon balance were reached for relative humidity between 25 and 75%. For a NiO-enriched alumina catalyst, Wu et al. derived the desorption activation energies of H2O and toluene and found that the adsorption of toluene is considerably weaker compared to H2O [183]. In 2016, Qin et al. published two papers on the use of Ag/Mn type of catalyst deposited on an alumina support and compared the abatement efficiency and COx selectivity to the use of Ag or Mn as such [42, 184]. Both parameters were found to be substantially enhanced for the combined catalyst, which, according to the authors, could be attributed to the combination of a better toluene adsorption on Ag+, while the metallic silver was held responsible for assisting the manganese oxide in decomposing the adsorbed VOC. Furthermore, it was shown that the impregnation sequence of either catalyst plays an important role on the type of by-products formed.

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Fig. 6.5 Ozone concentration for different catalyst compositions [186]

Xu et al. performed similar studies but focused on the use of SBA-15 as support instead of the alumina [185, 186]. They also link the improvement in abatement efficiency and carbon balance to the π-complexation between the silver ions and the toluene double bonds (Fig. 6.5). Besides the variation in catalyst composition, also the feeding method was changed from a continuous mode to an on/off system, resulting in an alternated adsorption/decomposition mode, which lead to a more efficient abatement of the target VOC. FT-IR and GC-MS analysis showed that the generation of other carbonaceous intermediates forms the basis for this altered behavior. For PPC systems, similar conclusions were found: when the relative humidity is increased, the abatement efficiency of the PPC system decreases [36]. For a palladium-enriched alumina oxide, the decomposition efficiency was found to be more than 90% for intermediate relative humidity. For extremely dry or humid air, the conversion efficiency drops to 37%. The lowered abatement efficiency is mostly contributed to changes in Van der Waals interactions. The use of MnPO4, Mn–SAPO–11, and Mn–APO–5 as catalysts for a system in a PPC configuration at 400  C was studied by Magureanu et al. [72]. Without the application of a plasma discharge, no catalytic activity was noted for the system, which stands in sharp contrast to the hybrid system used, where a pronounced synergetic effect was observed. Although not tested, the authors expect that converting to an IPC system would further increase the abatement efficiency, as the active species generated in the discharge would, under normal conditions, lead to an increased oxidation efficiency of the catalyst surface. Huang et al. also studied a PPC system, using a TiO2/Al2O3/Ni foam as catalyst [102]. Different active species were followed during the runs, and the catalytic formation of O3 was found to be the principal process for toluene abatement,

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activated oxygen being the predominant species interacting with the toluene, as described in paper [187]. Besides CO2, formic acid, benzaldehyde, benzene, acetic acid, and 2-methylamylene were found within the waste stream leaving the catalytic system. Others also studied the use of manganese oxide and cobalt oxide-coated metal fibers (MF), incorporated as the inner electrode, for the abatement of toluene (100 ppm) [67]. On bare MF, a 50% CO2 selectivity was noted at an energy density of 235 J/L. Upon the addition of the catalyst, the selectivity increased to 80%, and a near to 100% decomposition efficiency could be reached. After a 3-hour run, the reactor was scanned for polymeric deposits, but those were not found, indicating that the electrodes do not lose any activity during toluene abatement. The literature on toluene abatement is extensive, and not all papers found were included in the above discussion. The use of other catalysts (combinations) and supports such as Fe2O3, MnOx, CoOx, Cu, Ti, BaTiO3, TiO2, AgO, Ag, Pt, Ni, V, cordierit, zeolites, and honeycombs was also mentioned in literature. Abatement efficiencies were situated between 45 and 95%, depending on the plasma configuration (PPC vs IPC) and energy density used (2.5–1000 J/L) [20, 40, 41, 58, 60, 67, 68, 75, 137, 188–200].

6.3.2

Benzene

Benzene, one of the most common derivates from crude oil, is produced in mass both for its use as an industrial solvent as well as the starting material for a broad spectrum of more complex organic molecules. Furthermore, thanks to its high octane number, it is an important compound for gasoline. Despite its widespread use, benzene is a notorious carcinogenic directly linked to leukemia and bone marrow failure, and already since 1948 the safe work concentration has been set to 0. Several NTP reactor configurations and catalyst compositions for the abatement of benzene have been tested by Kim et al. The ferroelectric material used in a barium titanate packed-bed reactor was swapped for titanium dioxide pellets as such or in combination with silver or platinum [201]. Temperature for the oven encapsulating the NTP system was set at 100  C. The catalytic activity for the different catalyst combinations was found to be Ag/TiO2 > TiO2 > Pt/TiO2. CO2 selectivity was also found to be highest for the Ag containing catalyst (+15%) compared to the packedbed barium titanate reactor. Analysis of the by-products revealed only the presence of CO and CO2, which was further confirmed by the carbon balance. Varying the catalyst concentration and/or the pollutant residence time had no discernable effects besides a slight reduction in N2O formation, whereas the adjustment of the energy density had a more pronounced effect on the abatement efficiency [189, 202]. For a sufficiently low energy density, small amounts of formic acid were found as by-product.

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In a third study, the effect of silver concentration on the benzene oxidation efficiency was analyzed [203]. An increase of the metal additive did not influence the pollutant breakdown efficiency as such, but it is a crucial factor in the oxidation of intermediates adsorbing on the TiO2 surface, as indicated by the CO2 selectivity and carbon balance. Yet another study focused on unraveling the activation mechanisms for the Ag/TiO2 catalyst [49]. As mentioned earlier in the section on photocatalysis, the effect on the abatement efficiency of the UV light generated by the plasma is negligible. Also variations in temperature and dilution gasses (N2, Ar) did not give cause to any notable changes. The authors therefore concluded that the in situ decomposition of O3 by the Ag-enriched photocatalyst at higher energy density is the main factor influencing the oxidation of benzene. Kinetic studies showed a zero-order relation to benzene concentration, further validating these conclusions. Long-duration testing revealed that the catalyst can withstand deactivation for runs up to 150 h [189]. In a final study, other types of catalysts (γ-Al2O3, zeolites, TiO2) were tested in combination with a cycled system of adsorption and oxygen plasma, similar as what was done by Song et al. and Ogata et al. [13, 37, 70]. The progressive oxidation of benzene was studied for a systematic variation of the oxygen partial pressure between 0 and 80%. As to be expected, a higher oxygen partial pressure greatly enhances the abatement efficiency as well as the CO2 selectivity, regardless of the applied catalyst. To prevent the formation of NOx during the cycled plasma catalysis, regeneration has to occur with pure oxygen. A likely reaction mechanism, as proposed by Kim et al., suggests that the decomposition of such target VOCs primarily occurs through the surface interactions with the catalysts (see Fig. 6.6) [204].

Fig. 6.6 Plausible mechanism for IPC for VOCs on various catalysts [13]

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Ogata et al. used a hybrid packed-bed reactor filled with a mixture of aluminum oxide and barium titanate and compared the benzene decomposition efficiency to a barium titanate reactor as such and a barium titanate packed-bed reactor with the aluminum oxide position downstream of the IPC reactor [37]. For all the important process parameters (energy efficiency, N2O suppression, CO2 selectivity), the hybrid system gave the best results. The adsorption of benzene on the aluminum oxide combined with subsequent surface decomposition and gas phase reaction is believed to be, according to the authors, the primordially mechanism for the stimulated destruction efficiency of benzene. When applied in a cyclic process of adsorption and plasma activation, a further reduction in energy consumption could be obtained. In a follow-up study, the barium titanate was replaced with different metal supports including Co, Cu, Ag, and Ni, giving cause to a minor increase in COx selectivity and N2O suppression [71]. Another study from the same group describes the use of a zeolite combined with barium titanate in an IPC configuration for the abatement of diluted waste streams containing benzene [38]. An improved decomposition efficiency and COx selectivity were reported and linked to the higher adsorption capacity of the zeolite. Surprisingly, the decomposition efficiency of the benzene molecules adsorbed on the outer surface of the zeolite was higher compared to those adsorbing inside of the porous material. In 2003, Ogata et al. continued expanding the study by analyzing the influences of barium titanate pellet size and the effects of changing mixing ratios between the titanate and the catalyst, adsorbent, or zeolite [81]. With the exception of pellet sizes larger than 2 mm, which led to sparking, the plasma energy was not influenced by pellet size. Overall, it was found that a larger pellet size leads to a higher energy density near the contact points of the pellets. These results point out the need for hybrid techniques to optimize the catalytic process. In a more recent paper, Fan et al. performed a similar study, using Ag/HZSM-5, a metal supported zeolite as the catalytic component for a cycled IPC system [205]. Results showed excellent oxidation of the benzene at moderate discharge powers and at a low energy consumption (3.7  103 kWh/m3). Long-term experiments showed a negligible deactivation of the catalyst. Jiang et al. studied the use of a Ag/Ce oxide catalyst in different reactor configurations, as shown in Fig. 6.7 [206]. Surprisingly, a PPC configuration showed a higher abatement efficiency and improved CO2 selectivity of benzene compared to an IPC configuration. The combination of Ag and Ce species with certain proportion facilitated the surface lattice of the catalyst and increased the formation of surface adsorbed oxygen, which played a key role in the plasma-catalytic reactions and significantly improved benzene degradation. PPC processes were found to be more effective at decomposing O3 and destroying benzene due to the ability of catalysts to adequately decompose NTP generated ozone, especially when the catalysts were packed downstream. The use of photocatalysis for benzene abatement is also a popular route of investigation [48]. Glass beads were coated with variations of γ-Al2O3, examining the effects of pore volume, surface area, and pore diameter. The use of highly porous alumina had a pronounced positive effect on the benzene decomposition and the degree of mineralization.

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Fig. 6.7 Effect of plasma catalysis configurations on (a) benzene degradation efficiency and (b) CO2 selectivity as a function of SIE. Condition: Ag0.9Ce0.1/γ-Al2O3 was used as the catalyst; gas relative humidity was 0% [206]

Others used different sheet type catalysts (TiO2, V2O5/TiO2, and Pt/TiO2) deposited on the DBD layer of the electrodes. The efficiency in benzene abatement was as followed: TiO2 < Pt/TiO2 < V2O5/TiO2 [207]. Both an improvement in mineralization degree and a strong reduction in N2O production were noted and this for all catalysts used. The mechanism driving the benzene decomposition can, according to the authors, be attributed to the combination of UV in combination with high energy electrons, both generated by the plasma discharge.

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Futamura et al. filled a DBD system with a combination of manganese dioxide, titanium dioxide, and a titanium dioxide-silica gel [63]. The manganese dioxide, responsible for the formation of active oxygen out of adsorbed O3, would be the main driving force for the oxidative destruction of benzene. Hu et al. combined the use of TiO2 and MnOx catalysts on a zeolite support in an IPC configuration [208]. Compared to the use of either the titanium oxide or the manganese oxide, a higher abatement efficiency of the target VOC as well as an improved COx selectivity could be achieved. The authors attribute this synergetic effect to the charge transfer between Ti4+ and Mn4+, which resulted in an effective separation of photogenerated electrons and holes. This process contributed to the production of hydroxyl radicals, an essential component for the oxidative decomposition of benzene. Zhu et al. examined the influence of humidity on the abatement of benzene, using an IPC system filled with titanium dioxide-coated Raschig rings [139]. As was to be expected, an increased humidity reduced the decomposition efficiency. The influence of temperature and catalyst positioning (PPC vs IPC) were studied by Harling et al., using an γ-Al2O3 support combined with silver [165]. In comparison with thermal catalysis, a significant increase in NOx concentration could be detected, and a progressive increase of the greenhouse gas was found at higher temperatures. For both the IPC and PPC configurations, a more complete oxidation of benzene was found at lower temperatures. Other types of catalysts used in an IPC or PPC system configuration include, but not limited to, Kr/I2, silica gel, and TiO2 sol-gels in combination with Pt, Ni, Pd, and ferrierite. Most of which were used in combination with a support structure such as barium titanate and titanium dioxide of aluminum oxide [13, 16, 40, 110, 122, 172, 201, 209–214].

6.3.3

Trichloroethylene

TCE is a commonly used solvent in industry but is considered highly controversial, as it is a known carcinogenic and nonhuman health hazard. Therefore, its abatement is one of the most studied processes in IPC and PPC. Oda et al. studied the use of manganese oxide in a PPC configuration [96]. The effect of direct processing of the contaminated waste stream was compared to indirect processing, where plasma-treated clean air was mixed with the waste stream. Results show that manganese oxide is sufficiently effective to improve the abatement efficiency in both cases, as it is a known ozone dissociator, generating in both cases a sufficiently high concentration of oxygen radicals to efficiently break down the TCE molecular structure. Also, Han et al. studied the synergetic effect of PPC and MnO2 on the direct and indirect processing of TCE-containing waste streams [215]. The main component generated during the direct processing of the waste stream is dichloroacetylchloride (DCAC), a result from the collisions between plasma species and O2. Compared to the indirect process, the degradation efficiency is higher due to the oxidation of the rest-TCE into tricholoracetaldehyde by atomic oxygen generated

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from the dissociation of ozone at the MnO2 catalyst surface. When increasing the energy density from 120 J/L to 400 J/L, the COx yield was further increased from 35% to 98%. Oda et al. also studied IPC, using the photocatalytic TiO2 [216]. The effect of initial TCE concentration, sintering temperature for TiO2, and pellet size on the TCE abatement performance were investigated. When the sintering temperature for TiO2 was changed from 1100  C to 400  C, the breakdown voltage needed to generate the NTP was significantly lowered. The authors suggested that the electric field was disturbed by the non-uniformity of the disk-like pellets, resulting in a more concentrated electric field at the contact points of the pellets. This gave cause to the formation of contact point discharges, which decreased the breakdown voltage and thus improved the overall energy efficiency of the system. A decrease of the pellet size affected the gas flow distribution, resulting in an incomplete filling of the active area with plasma. Magureanu et al. used a DBD system which consisted of an inner electrode fabricated from sintered MF and coated with different transition metal oxides [64]. CO2 and CO selectivity were studied for a range of energy densities and they found that the MF as such reached a CO2 selectivity of 25%. Though, when combined with a manganese oxide coating, selectivity increased to 60%. Unfortunately, the use of manganese oxide in combination with the MF did not decrease the selectivity toward CO. The explanation as to why this change in selectivity could be observed was again assigned to the production of atomic oxygen from ozone adsorbed on MnO2. The active oxygen is considered to be responsible for the enhanced oxidation of TCE, thus leading to a higher selectivity toward CO2 [51, 64, 68]. X-Ray photoelectron spectroscopy (XPS) analysis of the catalyst surface after running the system showed that both Fe and Mn did not change their oxidation state. In terms of elemental composition, an enrichment of Fe was found on the catalyst surface, indication of an active reorganization of the Mn on the surface during the process run. Besides the enrichment of Fe, also small traces of Cl were detected, by-products of the TCE abatement, that bonded onto the catalyst surface. A study performed within the same research group focused on the use of SBA-15 embedded with gold nanoparticles for PPC [217]. Gold concentration was varied systematically, and the effect on CO2 selectivity was analyzed. Surprisingly, the lowest concentration of gold (0.5 wt%) resulted in the best selectivity. Similar to the manganese oxide, the SBA-Au system is able to generate oxygen radicals out of dissociated ozone, which in turn effectively decomposes the TCE. The authors suspected that the isolated Au+ acts as the active catalytic site. Subrahmanyam et al. looked for the existence of activated nitrogen species within their DBD discharge, using UV-VIS spectroscopy [51]. Measurements taken in the 250–500 nm wavelength region showed the emission of excited nitrogen, within the desired UV range of their used photocatalyst. Therefore, the authors suggest that the increased activity of the photocatalyst can be attributed to the presence of the UV light generated by the plasma as well as the (re)activation of the catalyst by the various active species present within the discharge zone.

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At Ghent University, Morent et al. reached a more complete oxidation at a reduced energy cost using an IPC system filled with cylindrical TiO2 pellets, which they assigned to the efficient adsorption of TCE onto the catalyst surface, thus considerably increasing the residence time within the discharge zone [52]. From the same group, Vandenbroucke et al. performed several studies using a PPC system driven by a DC glow discharge combined with either a Pd/γ–Al2O3 or a Pd/LaMnO3 catalyst located in a downstream oven [82, 218]. At 100  C, the PPC system could be operated in a synergetic regime, resulting in an increase in abatement efficiency of 12–22% more TCE compared to multiplication of the theoretically expected decomposition ratios [153]. Dinh et al. used the same PPC configuration but combined it with a manganese oxide catalyst. Improved COx yields were detected for the abatement of TCE compared to the plasma alone system [219]. This result was mainly attributed to the capacity of the catalyst to convert the ozone generated by the plasma into atomic oxygen, thus resulting in the complete oxidation of the target VOC. In a similar study, MnOx was combined with Ce, resulting in a lowered poisoning of the catalyst by the chlorinated by-products compared to the other catalysts used for this setup [220]. Vandenbroucke et al. have also defined a what they call a “synergy factor” in their work on plasma–catalytic decomposition of TCE with a PPC reactor [221]. This synergy factor equals to the ratio of the degree of TCE destruction by plasma catalysis to the sum of the degree of TCE destruction by plasma alone and the degree of TCE destruction by catalyst alone. A synergistic system is a system with a synergy factor greater than one. In their work, this factor ranges from 0.78 to 4.78 by varying the operating parameters indicating that synergy is only observed under specific conditions. According to Whitehead, this measure deserves adoption as it is an unambiguous indication of synergy [222]. Besides the use of catalysts discussed here, a number of other catalysts have been combined with NTP (DBD and DC) for the abatement of TCE. These include, but are not limited to, cobalt oxide, vanadium oxide, tungsten oxide, palladium, Cu– ZSM–5, and LaMnO3+δ [68, 82, 92, 96, 127, 153, 221, 223]. For most, the destruction efficiency of TCE is above 95%, but the energy efficiency varies between 50 and 670 J/L. For a more elaborate review on the abatement of TCE, using IPC and PPC in combination with the previously mentioned catalyst, the reader is referred to the review by Vandenbroucke et al. [224].

6.3.4

Other VOCs

Besides toluene, benzene, and TCE, a number of other VOCs have been studied as models for IPC and PPC systems. Although they are not the focus of this chapter, they are listed in Table 6.1, thus giving the reader a complete overview on the available literature for plasma catalysis.

6 Plasma-Catalytic Removal of VOCs Table 6.1 Published papers on removal of other VOCs with plasma catalysis

6.4

VOC Methane Ethylene Propane Propene Acetylene Cyclohexane Styrene Xylene Methanol Ethanol Acetaldehyde Buteraldehyde Formaldehyde Diethylether Acetone Dichloromethane Tetrachloromethane Chlorobenzene

165 Reference [16, 79, 125, 225–229] [230, 231] [70, 83, 87] [83] [34, 99, 101, 232, 233] [234] [84, 170, 189, 235–237] [43, 95, 189, 238–244] [245, 246] [247] [45, 248–254] [255] [44, 123, 171, 256–258] [259] [18, 94, 97, 131, 247, 260–264] [31, 77, 265–267] [80, 164, 267] [268]

Conclusion and Outlook

In this chapter, a thorough overview has been given on the abatement of some of the most common VOCs through plasma catalysis. As mentioned in the introduction, there still remain a lot of unknowns for the exact mechanisms driving the VOC decomposition. Yet, depending on the target VOC, certain trends were found for the influence of relative humidity, temperature and initial concentration on the decomposition efficiency, and COx selectivity. Also, for some catalysts or catalyst combinations, the principal decomposition mechanisms could be identified, (mostly) independent of the target VOC. Still, more fundamental research is required to identify all essential interactions between the mixture of active plasma species, the (adsorbed) VOC, and the catalyst and their relative importance. For the near future, a shift toward the abatement of VOC mixtures will become of greater importance if plasma-assisted catalysis wants to become a valid alternative for the current industrial solutions.

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Chapter 7

Plasma-Catalytic Decomposition of Ammonia for Hydrogen Energy Yanhui Yi, Li Wang, and Hongchen Guo

7.1 7.1.1

Introduction Hydrogen Energy

The energy and environmental crises have become a common concern in our human society. The use of hydrogen as a kind of sustainable and environmental-friendly energy can not only reduce human dependence on fossil fuels but also reduce greenhouse gas emissions [1]. Due to the oil crisis of the last century, countries around the world are committed to the development of the hydrogen economy. In 1993, Japan put forward the “sunshine plan” and proposed that the key to solve the problem of clean energy is by promoting the use of hydrogen energy. At the beginning of the twenty-first century, the United States issued the “blueprint report” on the development of hydrogen energy. In 2003, the European Union published the “future prospect report” of hydrogen energy and fuel cells. Between 2011 and 2015, China also increased the investment for the research and development (R & D) of hydrogen energy and fuel cells in the “12th Five-Year science and technology plan.” Clearly, hydrogen energy has been considered as a “future fuel”; its development and utilization has become one of the world’s development trends for the future [2]. So far, the direct application of hydrogen on fuel cell vehicles has been limited by hydrogen storage technologies, which refer to hydrogen storage vessels and Y. Yi · H. Guo (*) State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning, China L. Wang State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning, China College of Environmental Sciences and Engineering, Dalian Maritime University, Dalian, Liaoning, China Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, UK © Springer Nature Switzerland AG 2019 X. Tu et al. (eds.), Plasma Catalysis, Springer Series on Atomic, Optical, and Plasma Physics 106, https://doi.org/10.1007/978-3-030-05189-1_7

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hydrogen storage materials. In 2015, the US department of energy proposed requirements for hydrogen storage technology [3]: (i) (ii) (iii) (iv)

High hydrogen content (≧9 wt.% or ≧81 g/L) Low cost (≦67 $/kgH2) Low hydrogen loss (≦0.12%/day) Feasible transportation temperature (
40 to 80  C) and transportation pressure (≦10 MPa)

Nevertheless, the current status is that: (i) The gas-state hydrogen storage vessels not only need high pressure (20–70 MPa), but the hydrogen content is also too low (≦1 wt.%); (ii) The liquid-state hydrogen storage vessels have high costs in order to liquefy hydrogen and inevitably lose a lot of hydrogen (2–3%/day) because of the low boiling point of hydrogen (
252  C). (iii) The dehydrogenation temperature and recycling stability of hydrogen storage materials also need to be improved to meet the requirements of practical application [4]. Recently, Pandith and Dahari reviewed the research progresses on hydrogen storage materials [5, 6]. On-site hydrogen production is another alternative for application of hydrogen energy on fuel cell vehicles. Currently, the feed stocks for on-site hydrogen production mainly include hydrocarbons [7, 8], alcohol [9, 10], biomass [11, 12], water [13, 14], and ammonia [15, 16]. However, the hydrogen production from hydrocarbons, alcohol, and biomass inevitably produces CO2 and CO, which not only causes the deactivation of the platinum electrode in the fuel cell but also causes the greenhouse effect. Water electrolysis is a clean and pollution-free hydrogen production method, but it can not be used in fuel cell vehicles. The reason is that the H2/O2 fuel cell reaction is the reverse reaction of H2O electrolysis, which means no surplus energy can be used to drive the car. The hydrogen production by ammonia decomposition, however, does not contain any greenhouse gases or pollutants. In addition, NH3 features a low liquefied pressure, high hydrogen content, low cost and wide availability, meaning it is an excellent hydrogen carrier. Thus, on-site NH3 decomposition to supply hydrogen for fuel cell vehicles has drawn more and more attention. Recently, Enrique G. B. and Torrente-Murciano L. reviewed the research progress of NH3 decomposition for hydrogen energy [17, 18].

7.1.2

Thermodynamics of NH3 Decomposition

NH3 decomposition reaction (7.1) is endothermic, and its standard molar enthalpy change (ΔH) is about 54.4 kJ/mol. The H2/O2 fuel cell reaction (7.2) is exothermic, and its ΔH is about
285.8 kJ/mol. One mole of NH3 can produce 1.5 moles of H2, that is, consuming 54.4 kJ energy (NH3 decomposition) can produce 428.7 kJ energy

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Fig. 7.1 Equilibrium conversion of NH3 in NH3 decomposition reaction as a function of temperature at atmospheric pressure [19]

(fuel cell reaction), which means that under ideal conditions, 87.3% of energy produced can be used to drive the vehicle. However, in the actual operation process, the energy efficiency of reactions 7.1 and 7.2 is always lower than 100%. Currently, the energy conversion efficiency of H2/O2 proton exchange membrane fuel cell (PEMFC) (reaction 7.2) is about 40–60%, which means that 1.5 mole H2 in PEMFC could produce 171.5–257.7 kJ surplus energy. In addition to improving the energy efficiency of PEMFC, efforts should also be focused on reducing the energy consumption of NH3 decomposition, because the less energy consumed in NH3 decomposition, the more energy can be extracted to drive the car. NH3 ðgÞ ! 0:5 N2 ðgÞ þ 1:5 H2 ðgÞ 54:4 kJ=mol

ð7:1Þ

H2 ðgÞ þ 0:5O2 ðgÞ ! H2 OðlÞ

ð7:2Þ


285:8 kJ=mol

Figure 7.1 shows the function of NH3 equilibrium conversion vs temperature at atmospheric pressure [19]. At the temperature of 573, 673 and 773 K, the equilibrium conversion of NH3 is about 95.8%, 99.1% and 99.9%, respectively. However, the optimal NH3 conversion in recent studies is much lower than the equilibrium conversion, especially when cheap metal catalysts were used. Therefore, there is still plenty of room to study the NH3 decomposition reaction to increase NH3 conversion at low temperatures as much as possible, especially at high space velocity.

7.1.3

Catalysts for NH3 Decomposition

7.1.3.1

Active Components

The active component is the most important factor in heterogeneous catalysis. The investigated catalysts for NH3 decomposition reaction mainly focused on noblemetal Ru catalysts [19–25], cheap-metal Fe catalysts [20, 26], Ni catalysts [20, 27–

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32], Co catalysts [33–37], as well as transition metal carbides and nitrides catalysts [20, 38–42]. Among these catalysts, Ru is the most active metal component for thermal catalytic NH3 decomposition. 1. Ruthenium Ru is the most active component for NH3 decomposition, and the B5 type Ru atom is the real active site of Ru-based catalysts [43, 44]. The catalytic activities are related to the grain size and morphology of the Ru particle. Zheng’s group found that the supported Ru cluster catalysts with an average particle size range of 1.9–4.6 nm exhibited high ammonia decomposition activity, and the best particle size is 2.2 nm [45]. Garcia’s group also obtained the same conclusion [44]. The activity of B5 site is also closely related to grain shape, and hemispherical Ru grain with grain size in the range of 1.8–3 nm exhibited the highest ammonia decomposition activity; while the flat and elongated Ru grain with a grain size of about 7 nm also exhibited high ammonia decomposition activity [43]. Although the Ru based catalyst in ammonia decomposition showed a high catalytic activity, it is not suitable for large-scale application because of its scarcity and high cost. 2. Iron Fe is a cost-effective catalyst, but the NH3 decomposition activity of the Fe-based catalyst was two orders of magnitude lower than that of the Ru-based catalyst. Nevertheless, it has drawn many people’s attentions since Fe is abundant and cheap. The highest NH3 decomposition activity of Fe-based catalysts was reported by Lu’s group [46]. They used the carbon material CMK-5 as a support and encapsulated Fe2O3 nanoparticles with grain size of about 6 nm in the channels of CMK-5, which can prevent migration and sintering of active components at high temperature. Using this Fe2O3@CMK-5 catalyst, NH3 can be completely decomposed with 60,000 ml/(gh) space velocity at 973 K; NH3 can also be completely decomposed with 7500 ml/(gh) space velocity at 873 K. In the NH3 decomposition process, Fe can be easily transformed into iron nitrides. Kowalczyk and co-workers found that both the metallic iron phase and iron nitrides phase can catalyze the decomposition of NH3 [47]. While Pelka and co-workers found that the catalytic activity of metallic iron phase is much higher than the nano iron nitrides site in NH3 decomposition, and the corresponding apparent activation energy was 68 and 143 kJ/mol, respectively [48, 49]. 3. Nickel Ni is a high-efficiency active component for ammonia decomposition. Ganley and co-workers, using Al2O3 as the support, found that the sequence of catalytic activity of NH3 decomposition is Ru > Ni > Rh > Co > Ir > Fe [50]. Yin and co-workers, using CNTs as the support, found that the sequence of catalytic activity of NH3 decomposition is Ru > Rh > Ni > Pd  Pt > Fe [19]. Liu and co-workers found that 89% NH3 conversion could be achieved over a Ni/SBA-15 catalyst at 823 K with a space velocity of 30,000 ml/(gh) [51]. Xu and co-workers found that 98.3% NH3 conversion could be achieved over a CeO2-modified Ni/Al2O3 catalyst at 823 K with a space velocity of 30,000 ml/(gh) [52]. Since Ni is much more

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abundant and cheaper than Ru, and its catalytic activity is relatively higher than some other transition metals, Ni-based catalysts provide a feasibility for NH3 decomposition in large-scale application. 4. Cobalt Co is also an efficient active component in NH3 decomposition, and its activity is higher than Fe catalysts [33, 34, 53]. Arabczyk and co-workers found that the activation energies of NH3 synthesis reaction over Co and Fe catalysts are 268 and 180 kJ/mol, respectively. However, the activation energy of NH3 decomposition reaction over Co and Fe catalysts are 111 and 138 kJ/mol, respectively [33]. In addition, some reports indicate that when Co is used as the second active component it can regulate the electronic properties of the bimetallic catalyst, which can weaken the strong interactions between metal atoms and N, resulting in acceleration of N atom recombination to improve the ammonia decomposition activity [54]. 5. Transition Metal Carbides and Nitrides In 1973, Levy found that tungsten carbide catalysts have similar catalytic behaviors to a noble metal Pt catalyst [55], since transition metal nitrides or carbides have similar electronic structures to noble metals [56]. This finding extended the application of transition metal nitrides and carbides in heterogeneous catalysis. So far, carbide and nitride catalysts used in NH3 decomposition include tungsten carbide [38, 57], molybdenum nitride [58, 59] and iron nitride [48, 49, 60]. Among the investigated transition metal carbides and nitrides, Mo2N exhibited the highest activity in thermal catalytic NH3 decomposition. In addition, introduction of Co into molybdenum nitride (CoxMoyN/gamma-Al2O3) can further improve the catalytic activity. The reason is that Co reduces the adsorption energy of N atoms on Mo active site [61].

7.1.3.2

Supports

Supports can influence the catalytic properties of active component metals via regulating the size, morphology and electronic properties of the metal particles. The reported supports of catalysts for NH3 decomposition mainly include carbon materials (activated carbon, carbon nanotubes, fullerene and carbon fiber) [62–64], zeolites (MCM-41 and SBA-15) [27], and metal oxides (MgO, Al2O3, SiO2, ZrO2 and TiO2) [65–67]. The specific surface area, acidity, alkalinity and electron transfer capability are the important properties of supports, which have a great influence on NH3 decomposition. Generally, acidic supports have an inhibitory effect on NH3 decomposition; however, alkaline supports have a lower activation energy for ammonia decomposition; a large specific area of support favors the high dispersion of metals. Therefore, supports with large specific surface areas, strong alkalinity, and high electron transfer capability are beneficial to the NH3 decomposition reaction. With regard to Ru catalysts, carbon materials, especially some carbon materials with high thermal stability and strong electrical conductivity, are the best supports.

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Regarding other transition metal catalysts, however, alkaline metal oxide and silicon materials exhibited better performance than some other support materials. 7.1.3.3

Promoters

Generally, promoters have a dramatical influence on heterogeneous catalysis. As literature reported, alkali metals, alkaline earth metals and rare earth metals usually play the role of catalyst promoter to accelerate the NH3 decomposition reaction. The functions of promoters mainly include: 1. Adjusting the electronic structure of the main active components as an electron promoter. Yin et al. reported that promoter modification has dramatically improved the catalytic activity of the Ru/CNTs catalysts, and the catalytic activities sequence follow K-Ru > Na-Ru > Li-Ru > Ce-Ru > La-Ru > Ca-Ru > Ru [66]. Rarog et al. also found that the catalytic activity sequence of the modified Ru/CNTs catalysts follow Cs-Ru > K-Ru > Ba-Ru > Ca-Ru [68]. These studies clearly indicate that, with regard to Ru catalysts, the electronegativity of the promoters is a key factor; the lower the electronegativity, the better the catalytic performance. The reason is that the promoters with low electronegativity can transfer electrons to Ru easier, which can weaken the bond energy of Ru–N, resulting in promoting the desorption recombination of the surface adsorbed N atoms and eventually improving NH3 decomposition activity. 2. Improving the dispersion of the metal and inhibiting the sintering of nano metal particles (Ru, Ni, etc.) at high temperature as a structure promoter [52, 66, 69]. 3. Adjusting the interaction between the active metal component and the support. When the Ni/Al2O3 catalyst was used in NH3 decomposition, strong interactions between Ni and Al lead to the formation of nickel aluminum spinel, which strongly inhibited the reduction of Ni, and thus resulted in decrease of NH3 conversion [52]. While CeO2 modification has weakend the interaction between Ni and support Al2O3, which improved the reducibility of NiO, leading to enhancement of NH3 conversion [52]. CeO2 modification has also improved the catalytic performance of the Ni/SBA-15 catalyst in NH3 decomposition [69]. Some experimental results of NH3 decomposition by heterogeneous catalysis are shown in Table 7.1. It can be seen that Ru catalysts exhibit excellent activity in NH3 decomposition, even at relatively low temperature and high space velocity. However, the other transition metal catalysts can only show high NH3 conversion at high temperatures.

7.1.4

Mechanism of Catalytic NH3 Decomposition

The methanism of NH3 decomposition is more complicated than that of NH3 synthesis. It is widely believed that NH3 decomposition on the catalysts surface is mainly composed of a series of step-by-step dehydrogenation processes, as shown in

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Table 7.1 Studies of hydrogen production from ammonia decomposition by thermal catalysis

Catalysts LiNH2-Ru/ MgO Ru-Ba(NH2)2 K-Ru/MgOCNTs Microcapsularlike Ru@SiO2 Fe2O3/CMK-5

NH3 conversion (%) 100 –

H2 productivity (mmol/gmin) 68.3

99

8.07 66.3

100

33.5

100

66.9

Cs-Fe@microSiO2 CeO2-Ni/Al2O3

100

33.5

98

32.9

Co/MgO-La2O3 5% CoFe5 on CNTs WC

91 48

– 19.3

100

–

MoNx/SBA-15 MnN-Li2NH

100 –

– 37.5

Reaction conditions 547  C, 60,000 ml/(gh)

Reference [21]

400  C, 60,000 ml/(gh) 450  C, 0.1 g, 100 ml/min NH3, 60,000 ml/(gh) 550  C, 0.1 g, 50 ml/min NH3, 30,000 ml/(gh) 700  C, 0.025 g, 25 ml/min NH3, 60,000 ml/(gh) 650  C, 0.1 g, 50 ml/min NH3, 30,000 ml/(gh) 550  C, 0.1 g, 50 ml/min NH3, 30,000 ml/(gh) 550  C, 6000 ml/(gh) 600  C, 0.05 g, 30 ml/min NH3, 36,000 ml/(gh) 500  C, 0.05 g, NH3:He ¼1:54, 55 ml/min total gas flow 650  C, 15,800 ml/(gh) 500  C, 60,000 ml/(gh)

[25] [65] [70] [46] [71] [52] [37] [72] [38] [39] [40]

reactions 7.3, 7.4, 7.5, and 7.6, where (g) represents a gas-phase species and (ad) represents an adsorbed surface species. Then, the adsorbed H and N atoms may combine into adsorbed H2 and N2 molecule, respectively (reactions 7.7 and 7.8). The adsorbed H2 and N2 molecule eventually desorbed from catalyst surface (reactions 7.9 and 7.10). NH3 ðgÞ ! NH3 ðadÞ

ð7:3Þ

NH3 ðadÞ ! NH2 ðadÞ þ H ðadÞ

ð7:4Þ

NH2 ðadÞ ! NHðadÞ þ HðadÞ

ð7:5Þ

NHðadÞ ! NðadÞ þ HðadÞ

ð7:6Þ

2NðadÞ ! N2 ðadÞ

ð7:7Þ

2HðadÞ ! H2 ðadÞ

ð7:8Þ

N2 ðadÞ ! N2 ðgÞ

ð7:9Þ

H2 ðadÞ ! H2 ðgÞ

ð7:10Þ

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Species NH3 NH2 NH

Bond H-NH2 H-NH H-N

Dissociation energy kcal/mol kJ/mol 107.6 450.2 93.0 389.4 78.4 328.0

eV 4.7 4.0 3.4

With regard to the mechanism of catalytic NH3 decomposition, the rate controlling step is considered to be either the breaking of the first N–H bond of NH3 molecule (reaction 7.4) or the recombinative desorption of adsorbed N atom (reactions 7.7 and 7.9). As shown in Table 7.2, the dissociation energy for the N–H bond of NH3, NH2 and NH species reduces successively, which means that the breaking of the first N–H bond of NH3 molecule requires the highest energy. If the dissociation energy of the first N–H bond of NH3 molecule (Edis) is higher than the energy of Nad atoms recombinative desorption (Edes), i.e., Edis > Edes, the breaking of the first N–H bond of the NH3 molecule (reaction 7.4) is the rate controlling step. On the contrary, in the case of Edes > Edis, the recombinative desorption of adsorbed N atom (reactions 7.7 and 7.9) is the rate controlling step. Many researches have demonstrated that the breaking of the first N–H bond of NH3 molecule (reaction 7.4) is the rate controlling step over Ru, Ir, Pd, Pt and Cu Catalysts [50, 73, 74]; however, on the surface of Fe, Co, and Ni catalysts, the recombinative desorption of adsorbed N atom (reactions 7.7 and 7.9) is the rate controlling step [50, 75, 76]. The core issue of NH3 decomposition is how to accelerate the rate controlling step, which is mainly dependent on the strength of the M–N bond (M represents the active site of the metal catalysts). (1) A weak M–N bond is beneficial to the desorption of N atoms, but it is difficult to dissociate the N–H bond of an NH3 molecule; (2) a strong M–N bond is in favor of dissociating N–H bond, but strongly adsorbed N atoms are difficult to desorb from the surface of the catalyst; and (3) moderate M–N bond can not only benefit the dissociation of N–H bond but also favor the desorption of the strong adsorbed N atoms. Therefore, the ideal catalyst for NH3 decomposition should have a moderate M–N bond. Vlachos and co-workers, using first principles calculation and modeling, studied the NH3 conversion versus the nitrogen binding energies with different metal catalysts (Fig. 7.2). A volcano curve was established, and Ru with a moderate Ru–N bond energy of ca. 134 kcal/mol was found to have the highest activity, which is consistent with some experimental results [77]; when the metal–nitrogen bonding energy is too high or too low, it is unfavorable to NH3 decomposition This finding reveals that the binding energy of metal–nitrogen bond is a good reaction activity descriptor for catalytic NH3 decomposition. With regards to the catalysts Fe, Co and Ni, the recombinative desorption of the strong adsorbed N atoms is the bottleneck of NH3 decomposition, and the key issue for scholars has become how to find a way to solve this problem. Therefore, in addition to the innovation of catalyst preparation process, it is necessary to seek some new methods to help the catalyst to achieve efficient NH3 decomposition. The following will introduce the plasmacatalytic NH3 decomposition in a gliding arc discharge (GAD) reactor and dielectric barrier discharge (DBD) reactor, respectively.

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Fig. 7.2 Ammonia decomposition volcano curve. Ammonia conversion calculated from microkinetic modeling (circles, left axis) at 850 K for various transition-metal catalysts and experimental supported catalyst turnover frequencies (TOF, triangles, right axis) at 850 K plotted against the nitrogen binding energies QN(0) [77]

7.2

NH3 Decomposition in a Gliding Arc Discharge Reactor

Zhao and co-workers investigated NH3 decomposition in gliding arc discharge reactors [78–80]. A schematic of the experimental apparatus used for ammonia decomposition in GAD reactor is shown in Fig. 7.3. It consists of an ammonia gas supply system (ammonia tank and mass flow controller), a reaction system (reactor and power supply) and an analytical system (on-line gas chromatograph with thermal conductivity detector for N2 and NH3 detection). The discharge voltages, discharge currents and input powers were measured by the Tektronix DPO-3012 Digital phosphor Oscilloscope (Tektronix, America) with a power module. The reactors temperature was taken by thermocouples at two sites: one was fixed on the external surface of the reactor shell facing the electrode gap, and the other was bound to the lower end of the ground electrode. The discharge image (the size of arc filament) was obtained with a digital camera at exposure time 1/25 s (Sony H10, Japan). The active species of the ammonia plasma in the electrode gap were detected by the optical emission spectrometry under the conditions of 0.5 s exposure and 10 μm slit width (SP2758 Princeton Instruments, America). In order to calculate the ammonia conversion, the on-line mass spectrometer was used in advance to qualitatively analyze the products of ammonia decomposition in the reactor. It was confirmed that the products of ammonia decomposition in this type of plasma reactor contained only H2 and N2, and no hydrazine was found.

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Mass flow meter

Oscilloscope

Thermocouple

Reactor

NH3 lens

Insulating layer

ON OFF

Optical emission spectroscopy

GC/TCD

Power Supply

Fig. 7.3 The schematic illustration of the experimental apparatus and process for ammonia decomposition [80]

7.2.1

Influence of Reactor Configurations

The reactor configuration has a dramatic effect on NH3 decomposition [78]. Gliding arc discharge (GAD) can be triggered by alternating current plasma power in flat-flat, point-flat, tube-flat and tube-tube plasma reactors, as shown in Fig. 7.4. The flat-flat, point-flat, and tube-flat plasma reactors have the same high voltage electrode, i.e., stainless steel circular plate (3 mm thickness and 50 mm diameter); however, they have different ground electrodes. In the flat-flat, point-flat, and tube-flat plasma reactors, the ground electrodes consist of a stainless steel circular plate (3 mm thickness and 50 mm diameter), stainless steel bar (3 mm diameter) with sophisticated structure, and a stainless steel tube (3 mm diameter). In the tube-tube reactor, both the high voltage electrode and ground electrode are stainless steel tubes (3 mm diameter). In the flat-flat, point-flat and tube-flat plasma reactors, a dielectric barrier discharge (DBD) can be generated when the high voltage electrode is covered by an intact insulate dielectric. Otherwise, when the high voltage electrode was naked or was covered by an insulate dielectric with a hole in the center, the GAD would be generated. However, in the tube-tube reactor, only GAD can be generated. As shown in Fig. 7.5, the DBD plasma (labeled by intact dielectric barrier) exhibited the lowest NH3 conversion, no matter what kind of the reactors were used or how much power was inputted (20, 30, 40 and 50 W). In other words, the GAD plasma showed much

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Fig. 7.4 Scheme of configuration of the plasma reactor for NH3 decomposition [78]. (1) reactor shell; (2) fixing devices; (3) ground electrode; (4) high-voltage electrode; (5) dielectric

Fig. 7.5 The influence of reactor configuration and discharge mode on NH3 conversion [78]

higher NH3 conversion than the DBD plasma. In addition, when the high voltage electrode was covered by a dielectric with a hole (labeled by hole on dielectric barrier), higher NH3 conversion was obtained than that of naked high voltage electrode (labeled by without dielectric barrier). That is, in the flat-flat, point-flat and tube-flat plasma reactors, the NH3 conversion followed the sequence of hole on dielectric barrier > without dielectric barrier > intact dielectric barrier. However, at the condition of GAD plasma, the tube-tube reactor has the highest NH3 conversion in the investigated four reactors, and the NH3 conversion followed the sequence of tube-tube > tube-flat > point-flat > flat-flat.

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Mechanism of NH3 Decomposition in a GAD Reactor

In the mode of DBD plasma, the optical emission spectroscopy (OES) has been collected. The OES of NH3 DBD plasma was dominated by the band of the electronic excited NH3 molecule, i.e., 563.5–567 nm (Fig. 7.6). There were also some bands of N2 (300–500 nm) and NH (336.0 nm), and a line of H (656.3 nm), but their intensities were extremely weak. The attribution of these bands and the line are shown in Table 7.3. However, in the mode of GAD plasma, the OES (Fig. 7.7) was dominated by the line of H (656.3 nm), as well as the bands of NH (336.0 nm) and NH2 (661.9 nm) species. There were also some bands of N2 (300–500 nm), N2+ (391.4 and 427.8 nm) and H2 (400–700 nm) with weak intensity. The attribution of these bands and line are shown in Table 7.4. The most important information is that the band of NH3 species did not appear in the OES of NH3 GAD plasma (Fig. 7.7), but it was collected as the main peak in the OES of NH3 DBD plasma (Fig. 7.6), indicating different mechanism of NH3 decomposition in the modes of DBD and GAD plasma. The NH3 conversion of DBD plasma was much lower than that of the GAD plasma. The above mentioned OES data indicates that, in the NH3 DBD plasma, the decomposition of NH3 may be mainly achieved by the NH3 species, and the most

Fig. 7.6 The OES of DBD NH3 plasma

NH3∗

200 NH∗(Α3 Π→X3 Σ–)

tube-flat 26.0 W

Ha

Intensity

point-flat 26.5 W flat-flat 25.8 W

N2∗(C3 Πu → B3 Πg)

300

400

wire-cylinder 30.0 W 500

600

Wavelength (nm)

Table 7.3 Active species in DBD NH3 plasma [81–84] Species λ, nm

NH 336

N2 337max

Electronic transition

A3∏ ! X3S-

C3∏u ! B3∏g

NH3 563.5 and 567 continuum Schuster’s bands

Ha 656.3 2p2p03/2– 3d2D3/2

700

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Πu(3d2D → 2p2p2)

1000 NH∗(Α3 Π → X3 Σ–)

tube - tube 25.2 W Intensity

∗

1

˜ 2 B1) NH2 (Ã2A1 → X

1

N2 (B2 Σu → X2 Σg)

tubu - flat 26.1 W H2∗ point - flat 25.8 W N2∗ (C3Πu → B3 Πg)

flat - flat 26.4 W 300

400

500

600

700

Wavelength (nm)

Fig. 7.7 The OES of GAD NH3 plasma Table 7.4 Active species in GAD NH3 plasma [83–86] Species NH Hα N2

λ (nm) 336, 337 656.3 391.4, 427.8

Electronic transition A3∏ ! X3S
2p2p03/2–3d2D3/2 B2∑u ! X2∑g1

Species N2 H2 NH2

λ (nm) 300–500 400–700 661.9

Electronic transition C3∏u ! B3∏g – Ã2A1 ! X2B1

possible NH3 decomposition mechanism in DBD plasma is shown as the reactions 7.11, 7.12, and 7.13. e þ NH3 ! NH3  þ e 

ð7:11Þ

e þ NH3 ! NH2 þ H þ e

ð7:12Þ

e þ NH2 ! NH þ H þ e

ð7:13Þ

However, in the NH3 GAD plasma, the decomposition of NH3 may be mainly achieved by the NH2 and NH species, and the most possible NH3 decomposition mechanism in GAD plasma is shown as the reactions 7.14, 7.15, and 7.16. e þ NH3 ! NH2 þ H þ e

ð7:14Þ

e þ NH3 ! NH þ H2 þ e

ð7:15Þ

e þ NH3 ! NH þ H þ H þ e

ð7:16Þ

Generally, GAD plasma has a much higher electron energy and electron density than the DBD plasma. In the NH3 DBD plasma, NH3 was mainly activated into

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NH3 species, then the NH3 species was further activated by using an electron to dehydrogenate and generate NH2 and NH species. In the NH3 GAD plasma, however, NH3 was directly activated and transformed into NH2 and NH species by high energy electrons, which could be the reason why GAD plasma exhibited much higher efficiency in NH3 decomposition than the DBD plasma. As literature reported, there were many reactions that produce H2 and N2 from NH2 and NH species, but the fastest reaction is the self-disproportionation reaction (7.17) [82]. So, the H2 and N2, in both DBD and GAD NH3 plasma may be mostly generated from NH species through reaction 7.17. NH þ NH ! N2 þ H2

7.2.3

ð7:17Þ

Synergy of the Plasma and Electrode Catalysts in the Tube-Tube Reactor

GAD plasma in the tube-tube reactor was the most efficient path for NH3 conversion. In the tube-tube reactor, electrode materials were investigated, and it has been found that some metal electrodes also played the role of a catalyst for NH3 decomposition [79]. As shown in Fig. 7.8, when Ni and stainless steel (SS) electrodes were used in the tube-tube reactor, there were clear induction periods, during which NH3 conversion increased remarkably with time and remained constant after 100 min and 240 min, respectively. When Cu electrodes were used, however, a constant NH3 conversion appeared after the beginning of the discharge. To compare the performance of different electrodes in NH3 decomposition, data were taken after the induction period. As shown in Fig. 7.9, the ammonia conversion increased with the input power no matter which electrodes were used. At the same input power, however, the ammonia conversion varied with electrode materials and followed the sequence of Ni > SS > Cu. The catalytic function of the Ni and SS electrodes was investigated by Zhao and co-workers [79]. The external surfaces of both high-voltage electrode and ground electrode were defunctionalized by tight quartz annular tubes. As shown in Fig. 7.10, the defunctionalization of the external surface of Cu electrode had no influence on the NH3 conversion, indicating that the surface of Cu electrodes was inert. In other words, the NH3 conversion was contributed to only by gas-phase plasma reactions when the Cu electrode was used. In contrast, the defunctionalization of the external surface of Ni and SS electrodes caused remarkable decreases in NH3 conversion, which implied that the surfaces of Ni and SS electrodes had obvious catalytic activity for NH3 decomposition. Taking the Cu electrodes as a reference, the contributions to the NH3 conversion by the catalytic decomposition of Ni and SS electrodes were estimated to be 51.7% and 49.8%, respectively.

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Fig. 7.8 Time dependence of ammonia conversion on different electrodes [79]

Fig. 7.9 Comparison of the ammonia conversion obtained with different electrodes. (Data was taken after ammonia conversion induction) [79]

The electrode catalysis of the high-voltage electrode and the ground electrode were investigated, using the tube-tube reactors with Cu–Ni and Cu–SS electrodes, respectively. As shown in Fig. 7.11, when Cu was used as a high-voltage electrode and a ground electrode, respectively, and Ni and SS were used as the counter electrode accordingly, the NH3 conversion decreased significantly in all cases when compared with those double Ni and SS electrodes. Comparatively, the use of Cu as high-voltage electrode caused a bigger drop in NH3 conversion. This means that in the cases of double Ni and SS electrodes, the high-voltage electrodes had a higher catalytic activity than the ground electrode. However, it seemed that the relative catalytic activity of the high-voltage electrode varied with electrode material. For example, the catalytic activity ratio of the high-voltage electrode to the ground electrode was estimated to be about 3:2 for Ni electrodes and 2:1 for SS electrodes. The temperature difference between the high-voltage electrode and ground electrode

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Fig. 7.10 Influence of metal electrode external surface defunctionalization with quartz annular tubes on NH3 conversion. (Data was taken after NH3 conversion induction) [79]

Fig. 7.11 Influence of substituting Ni and SS electrode with a Cu electrode on the NH3 conversion. (Data was taken after ammonia conversion induction) [79]

in the SS electrode reactor is shown in Fig. 7.12. It clearly shows that the temperature of the high-voltage electrode was much higher than that of the ground electrode. A similar phenomenon also appeared with Ni and Cu electrode reactors. Thus, the different catalytic activity of the high voltage electrode and ground electrode could be mainly caused by the different temperatures. The essence of the induction period for NH3 decomposition in the case of both SS and Ni electrodes was investigated. The XRD patterns of the SS electrode samples with different discharge time during the induction period are shown in Fig. 7.13. It can be seen that the fresh SS electrode had the typical structure of austenitic stainless steel which was featured by the existence of (Fe, C) phase. During the induction period, the intensities of the austenitic stainless steel characteristic peaks decreased

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Fig. 7.12 The temperature of the SS electrodes in the tube-tube reactor detected using a thermal infrared imager [79]

Fig. 7.13 XRD patterns of SS electrodes sampled at different discharge times during the induction period [79]

with time. Meanwhile, new phases of Fe, Fe4N and Fe2N appeared. Specifically, the Fe phase appeared first of all, and its peak intensities notably increased from 10 min to 120 min and decreased by 240 min. Fe4N and Fe2N appeared after 30 min and 120 min, respectively, and their peak intensities increased thereafter in the induction period. As before mentioned, Fe and its nitrides had catalytic activity for ammonia decomposition in traditional heterogeneous reactors [46–49, 60]. Therefore, it is clear that the induction period of the SS electrode was actually a complex process featured by the reduction of the austenitic stainless steel (Fe, C) to Fe phase first, and

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Fig. 7.14 Effect of different discharge pretreatment on the induction period of SS electrodes in the decomposition of ammonia [79]. (N2 pretreatment (20 ml/min, 300 min); N2-H2 pretreatment (N2/ H2 ¼ 1:3, 80 ml/min, 300 min); NH3-He pretreatment (NH3/ He ¼ 1:1, 80 ml/min, 40 min); NH3-Ar pretreatment (NH3/ Ar ¼ 1:1, 80 ml/min, 40 min))

then the Fe phase was partially nitrided to Fe4N and Fe2N. The increase of the ammonia conversion during the induction period could be attributed to the catalytic activity of Fe and its nitride on the surface of electrode. Four kinds of discharge pretreatments have been employed for the SS electrodes, as shown by Fig. 7.14. It can be seen that the SS electrodes pretreated in nitrogen or in the mixture of nitrogen and hydrogen needed a similar induction period compared to the fresh SS electrodes when they were used for NH3 decomposition in the tubetube reactor. In contrast, the SS electrodes pretreated in the mixtures of NH3-Ar and NH3-He for 40 min could entirely eliminate the induction period in the decomposition of ammonia. It can be inferred that it was not nitrogen but ammonia that made the SS electrodes nitrided during the induction period. The XRD patterns of the Ni electrode samples with varied discharge times during the induction period are shown in Fig. 7.15. However, no obvious diffraction peaks related to Ni nitride were found, although the same induction period as the SS electrode was seen during ammonia decomposition. Using the on-line mass spectroscopy, it has been found that the used Ni and SS electrodes gave an obvious nitrogen desorption peak when they were treated by He GAD (Fig. 7.16). This experiment confirmed the existence of a small amount of nitrides on the surface of Ni electrodes, but it may not be identified by XRD due to small amount or small grains. In order to demonstrate this idea, the following experiments were carried out. First, the SS and Ni electrodes were treated in NH3 GAD for 300 min and 120 min, respectively. Second, the pretreated electrodes were then used for NH3 decomposition directly. Third, the pretreated electrodes were re-treated by He GAD for 60 min to desorb N. Fourth, the re-treated electrodes were subjected to NH3 decomposition. Figure 7.17 shows that the Ni electrodes and the SS electrodes exhibited similar NH3 conversion curves in each case. That is, after pretreated by NH3 GAD, the induction period has been eliminated; however, the induction period reappeared when the He-retreated Ni and SS electrodes were used. These comparative experiments

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Fig. 7.15 XRD patterns of Ni electrodes sampled at different discharge times during the induction period [79]

40 0.5 E-9A

30 W plasma on N2

Ion current (E-9A)

Input power (W)

20

0

–20

SS electrodes

Ni electrodes

N2

–40 10

15

20

25

30

Time on stream (min)

Fig. 7.16 On-line mass spectroscopy monitor of the He discharge desorption treatment of the Ni and SS electrodes that were pretreated by ammonia discharge [79]

assured that the nitridation should be related to the induction period of the Ni electrodes. Thus, the Ni nitride on the surface of the Ni electrode may have very high NH3 decomposition activity in view of its relatively small amount compared with the nitrides on the surface of SS electrodes.

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Fig. 7.17 Effects of ammonia-discharge and He-discharge treatments on the inductions of the Ni and SS electrodes [79]

7.2.4

Influence of Process Parameters in Tube-Tube Reactor

7.2.4.1

The Parameters of the Tube-Tube Reactor

In the tube-tube reactor, the electrode gap and electrode tube diameter had remarkable influence on NH3 conversion [80]. It can be seen from Fig. 7.18, when the electrode tube diameter was 2 mm, the ammonia conversion increased with the decrease of electrode gap from 10 to 1 mm. However, when the electrode tube diameter was 3 mm, the ammonia conversion increased with the increase of electrode gap from 1 to 4 mm, and then decreased with the further increase of the electrode gap from 4 to 10 mm. When the electrode tube diameter was 6 mm, the ammonia conversion increased with the increase of electrode gap from 1 to 10 mm. As mentioned in Sect. 7.2.3, the NH3 conversion in the tube-tube reactor was contributed to by both the gas-phase plasma decomposition reaction and the electrode-surface catalytic decomposition reaction. When SS electrodes were used, the contributions of the two aspects to the total NH3 conversion were about a 50/50 percentage under the conditions of 25 W input power, 40 ml/min ammonia flow rate, 4 mm electrode gap, and 3 mm electrode tube diameter [79]. However, the surface of Cu electrodes did not have any catalytic activity for NH3 decomposition. Therefore, Cu electrodes have been used to gain insight into the influence of the electrode gap and electrode tube diameter on NH3 conversion. The discharge image and the temperature of the tube electrode were attained. As shown in Fig. 7.19, the discharge was featured by a bright arc filament between the two electrodes. The area of the bright arc filament was considered as the effective plasma volume. This means that the gas-phase plasma decomposition of NH3 mainly occurred in the effective plasma volume where energetic electrons existed. The effective plasma volume would therefore be larger when the arc filament got longer

7 Plasma-Catalytic Decomposition of Ammonia for Hydrogen Energy

a

b1

100

100

φ = 2 mm

φ = 3 mm

60 d = 10 mm → d = 1 mm

40 20 0 10

15

20 Input power (W)

d = 1 mm d = 2 mm d = 3 mm d = 4 mm d = 5 mm d = 6 mm d = 7 mm d = 8 mm d = 9 mm d = 10 mm 25

30

100

80

NH3 conversion (%)

NH3 conversion (%)

80

b2

60

0

c

15

d = 4 mm → d = 10 mm

20

15

20

20

25 30 Input power (W)

35

40

100

25 30 Input power (W)

d = 4 mm d = 5 mm d = 6 mm d = 7 mm d = 8 mm d = 9 mm d = 10 mm 35

40

NH3 conversion (%)

60

0

d = 1 mm d = 2 mm d = 3 mm d = 4 mm

20

φ = 6 mm

80

40

d = 1 mm → d = 4 mm

40

φ = 3 mm NH3 conversion (%)

201

d = 10 mm → d = 1 mm

80

d = 1 mm d = 2 mm d = 3 mm d = 4 mm d = 5 mm d = 6 mm d = 7 mm d = 8 mm d = 9 mm d = 10 mm

60 40 20 0 15

20

25

30

35 40 45 50 Input power (W)

55

60

65

Fig. 7.18 Effect of electrode gap (d ) and electrode diameter (Φ) on NH3 conversion (double SS electrodes, 40 ml/min, 5 kHz, reactor diameter 8 mm) [80] Fig. 7.19 The influence of electrode gap (d ) and electrode diameter (Φ) on the effective plasma volume and electrode temperature (double Cu electrodes 40 ml/min, 22 W, 5 kHz, reactor diameter 8 mm) [80]

due to the increase of electrode gap. Thus, the gas-phase plasma decomposition of NH3 would be enhanced by the increase of the electrode gap. When the electrode gap was fixed, however, there was no obvious change in the size of the bright arc filament. That is, the effective plasma volume kept stable during the increase of the electrode tube diameter. Nevertheless, more and more NH3 molecules would

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bypass the effective area of plasma with increasing the electrode tube diameter. This means that the interaction between NH3 molecules and the energetic electrons would be less effective when a larger electrode tube was used. Figure 7.19 also indicates that, when the electrode tube diameter was fixed, the temperature of the ground electrode decreased with the increase of electrode gap from 1 to 10 mm. Similarly, when the electrode gap was fixed, the temperature of the ground electrode decreased with the increase of electrode tube diameter from 3 to 6 mm. These results indicate that the influences of the electrode gap and the electrode tube diameter on NH3 conversion were actually the combined effects of both the gas-phase plasma decomposition reaction and the electrode-surface catalytic decomposition reaction. This is because the increase of the effective plasma volume would result in the enhancement of the gas-phase plasma ammonia decomposition reaction, while the decrease of the electrode temperature (the temperature of both ground and high-voltage electrodes would increase or decrease at the same time) would lead to the weakening of the surface catalytic ammonia decomposition reaction. To verify the relationship between the effective plasma volume and the gas-phase plasma NH3 decomposition reaction, parallel experiments were carried out by using inert Cu tube electrodes (only gas-phase plasma decomposition of ammonia existed in Cu–electrode reactor [79]) instead of SS electrodes in the same reactor, and the electrode gap was regulated. Figure 7.20 shows that, in both 2 and 3 mm electrode tube diameters, the gas-phase plasma NH3 decomposition reaction was indeed strengthened by the increase of the electrode gap. It is interesting to note that, in the case of using a 2 mm electrode tube, the NH3 conversion monotonously increased with the decrease of electrode gap from 10 to 1 mm; while in the case of using 6 mm electrode tube, the NH3 conversion monotonously decreased with the decrease of electrode gap from 10 to 1 mm (Fig. 7.18). In the case of using a 3 mm electrode tube diameter, however, the NH3 conversion first increased and then decreased with the decrease of electrode gap

a

b

100

100 φ = 3 mm

φ = 2 mm 80

d = 1 mm → d = 10 mm

60

d = 1 mm d = 2 mm d = 3 mm d = 4 mm d = 5 mm d = 6 mm d = 7 mm d = 8 mm d = 9 mm d = 10 mm

40

20

0 15

20

25

Input power (W)

30

35

NH3 conversion (%)

NH3 conversion (%)

80

d = 1 mm → d = 10 mm d = 1 mm d = 2 mm d = 3 mm d = 4 mm d = 5 mm d = 6 mm d = 7 mm d = 8 mm d = 9 mm d = 10 mm

60

40

20

0 15

20

25

30

35

40

45

50

Input power (W)

Fig. 7.20 Effect of electrode gap (d ) and electrode diameter (Φ) on NH3 conversion (double Cu electrodes, 40 ml/min, 5 kHz, reactor diameter 8 mm) [80]

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from 10 to 1 mm and showed the maximum value for a 4 mm electrode gap (Fig. 7.18). This phenomenon is a reflection of the varying contributions of the gas-phase plasma decomposition reaction and the electrode-surface catalytic decomposition reaction. In the case of using a 2 mm electrode tube, the NH3 decomposition was dominated by the electrode-surface catalysis. As a result, the NH3 conversion decreased with the increase of electrode gap owing to the decrease of the electrode temperature (Fig. 7.18). In the case of using 3 mm electrode tube, the NH3 decomposition was determined equally by the gas-phase plasma decomposition reaction and the electrode-surface catalytic decomposition reaction [79]. Therefore, the NH3 conversion increased with a decrease of the electrode gap from 10 to 4 mm, and the NH3 conversion decreased with the further decrease of electrode gap from 4 to 1 mm. In the case of using 6 mm electrode tube, the NH3 decomposition was dominated by the gas-phase plasma decomposition due to the low electrode temperature as a whole. Consequently, the NH3 conversion decreased with the decrease of electrode gap owing to the decrease of the effective plasma volume.

7.2.4.2

The Discharge Frequency

The discharge frequency is directly related to the validity of gliding arc discharge. As shown in Fig. 7.21, when the discharge frequency decreased from 17 to 5 kHz, the duration of the discharge stage (the period between the breakdown voltage of gas and the voltage when the discharge current disappeared) gradually increased. This means that the discharge time increased with the decrease of discharge frequency. When the discharge was carried out at 17 kHz, the discharge time accounted for about 55% in one cycle of applied voltage. By contrast, when the discharge was carried out at 5 kHz, the discharge time increased to about 83%. As shown in Fig. 7.22, the discharge frequency is also directly related to the effective plasma volume and the electrode temperature, which increase with the decrease of discharge frequency. Consequently, both the gas-phase plasma decomposition and the electrode-surface catalytic decomposition of ammonia were enhanced. The NH3 conversion increased with the decrease of discharge frequency (Fig. 7.23). When the discharge frequency decreased from 17 to 5 kHz under the conditions of 40 ml/ min NH3 flow rate, 25 W input power, 8 mm reactor diameter, 3 mm electrode diameter, and 4 mm electrode gap, the NH3 to H2 energy efficiency increased from 3.8 to 5.7 mol/kWh. In addition, it has been found that thermal insulation has a dramatic influence on NH3 decomposition. Under the conditions of 40 ml/min NH3 flow rate, 20 W input power, 3 mm electrode diameter, and 4 mm electrode gap, the energy efficiency increased from 5.6 to 7.9 mol/kWh when the 8 mm diameter reactor was insulated [80].

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breakdown voltage of ammonia

the voltage when the discharge current disappear

5 KHz 7 KHz 9 KHz 11 KHz 13 KHz 15 KHz 17 KHz duration of the discharge stage

duration of the discharge stage

one cycle of applied voltage Fig. 7.21 The effect of discharge frequency on the duration of the discharge stage [80]

Fig. 7.22 The influence of discharge frequency on the effective plasma volume and electrode temperature (40 ml/min, 25 W, reactor diameter 8 mm) [80]

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Fig. 7.23 The influence of discharge frequency on NH3 conversion (40 ml/min, reactor diameter 8 mm, electrode diameter 3 mm, electrode gap 4 mm) [80]

7.3

NH3 Decomposition in a Dielectric Barrier Discharge Reactor

Wang and co-workers studied the plasma-catalytic NH3 decomposition in DBD reactors [87–89]. The scheme of the experimental setup for NH3 decomposition through combination of DBD plasma and catalysis is shown in Fig. 7.24. NH3 decomposition was performed in a DBD reactor at atmospheric pressure, with a catalyst bed in the discharge zone. The reactor had two coaxial bare-metal electrodes. The shell was a quartz tube (10 mm o.d.  8 mm i.d.), which served as the dielectric barrier. The outer ground electrode was an Al foil (0.1 mm thick) wrapped tightly around the surface of the quartz tube. The inner high-voltage electrode was a stainless-steel rod (2 mm o.d.; surface composition, Fe 68.5 wt%, Cr 19.9 wt%, Ni 8.1 wt%, Mn 2.0 wt%, Si 0.5 wt%, Cu 0.7 wt%, Ag 0.1 wt%, Al 0.1 wt%, and C 0.1 wt%), and it was installed along the axis of the quartz tube. An alternating current (AC) supply was used, and catalyst was embedded in the plasma zone. Before discharge, an NH3 feed (99.999% purity) was flushed through the reactor for 30 min to remove the air, and then the DBD plasma was generated by switching on the AC power supply. NH3 was decomposed to N2 and H2 by the DBD plasma and the catalyst. The products were analyzed using an on-line gas chromatograph equipped with a thermal conductivity detector for N2 and NH3 detection. The input power of the plasma reactor was measured using a digital oscilloscope (Tektronix DPO 3012 digital oscilloscope equipped with a Tektronix P6015A high-voltage probe and a Pearson 6585 current probe, USA).

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PC

ICCD

NH3

Mass Flow Controller

Thermal Couple

~ 4 cm

External Capacitor High Voltage Probe

Gas Cylinder

Exhaust Gas

High Voltage Power Supply GC AC

230 V

Fig. 7.24 Scheme of the experimental setup used for H2 generation from NH3 decomposition through combination of DBD plasma and catalysis [89]

7.3.1

Synergy of the DBD Plasma and Bulk Fe Catalyst

7.3.1.1

The Bulk Fe Catalyst

Wang and co-workers studied NH3 decomposition using a DBD plasma and a commercial bulk Fe catalyst which is used for industrial NH3 synthesis [87]. As shown in Table 7.5, the main component of the bulk Fe catalyst is FeO, and it also has many promoters. During NH3 decomposition, the catalyst was placed in the discharge zone of the DBD reactor.

7.3.1.2

NH3 Decomposition Results

When the fresh bulk Fe catalyst was used, the initial NH3 conversion was as low as that obtained with DBD plasma (~10%) along, and NH3 conversion then increased as time progressed, and it reached a maximum after about 6 h, as shown in Fig. 7.25. Figure 7.26 shows that, when the Fe catalyst was used after undergoing a 6-h plasma-catalytic NH3 decomposition (Fe-based catalyst) reaction, a dramatically enhanced NH3 conversion by plasma-driven catalysis was observed around 400  C. At 410  C, the plasma-driven catalysis, i.e., the combination of DBD plasma and Fe-based catalyst, gave a NH3 conversion higher than 99.9%, while the Fe-based

7 Plasma-Catalytic Decomposition of Ammonia for Hydrogen Energy Table 7.5 Analysis of fresh commercial Fe-based catalyst by X-ray fluorescence (XRF)

Component FeO K2O SiO2 SO3 MnO Cl NiO ZnO

Content (%) 94.4 0.166 0.477 0.235 0.0605 0.0762 0.0205 0.0117

Component Al2O3 V2O5 TiO2 MgO Lu2O3 CuO MoO3

207 Content (%) 2.19 0.835 0.446 0.968 0.0376 0.0282 0.0191

X-ray fluorescence: SRS-3400, Germany, 40 kW, 60 kV, 150 mA, 75 mm, 0.01  C Fig. 7.25 The role of the catalyst in the synergy obtained in ammonia decomposition by plasmadriven catalysis (NH3 40 ml/min, 2 g FeO, the reactor temperature increased to 500  C within about 5 min and then kept constant with input power 26 W, discharge frequency 12 kHz) [87]

Fig. 7.26 Influence of reaction temperature on the synergy between DBD plasma and an Fe-based catalyst for ammonia decomposition [87]

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Fig. 7.27 X-ray diffraction patterns of FeO after NH3 decomposition in plasmadriven catalysis mode [87]. (X-ray diffraction: D/MAX-2400, Japan, 12 kW, 0.01, Cu, λ ¼ 0.1541 nm)

catalyst alone and the DBD plasma alone only gave NH3 conversions of 7.4% and 7.8%, respectively. The plasma-driven catalysis decreased the complete decomposition temperature by 140  C compared to the Fe-based catalyst alone and increased the energy efficiency and rates of hydrogen formation more than ten times. The above results demonstrate a synergy between the bulk Fe-based catalyst and the DBD plasma in NH3 decomposition. NH3 conversion gradually increased from 10% to 99% (Fig. 7.25) when a fresh commercial bulk Fe catalyst was placed in the plasma reactor at a fixed input power (26 W). Figure 7.27 seemingly indicates that FeO was transformed into Fe/Fe3N/ Fe4N. The FeO, however, might be transformed to metallic Fe together with H2O during on-site NH3 decomposition reaction process. The nitride could be formed during the cooling process. The XRD analysis of the catalyst after the reaction was done after turning off the plasma and keeping the catalyst in NH3 flow until the catalyst was totally cooled to room temperature. When the DBD plasma was turned off, part of the metallic Fe inevitably adsorbed N to form Fe3N/Fe4N in NH3 flow. The activity was obviously influenced by the structure of the catalyst; such a phenomenon reflects the crucial role of catalyst in the synergy of plasma-driven catalysis. Therefore, it can be inferred that the majority of NH3 molecules were decomposed on the surface of catalysts.

7.3.2

Synergy of the DBD Plasma and Supported Metal Catalysts

7.3.2.1

Characterization of the Supports and Supported Catalysts

The supported Fe, Co, Ni and Cu catalysts were prepared using an incipient wet impregnation method [88]. Briefly, the metal (Fe, Co, Ni and Cu) nitrate (30 wt% metal in the final catalyst) was dissolved in deionized water. The supports (TS-1

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um zeolite, TS-1 nm zeolite, NaZSM-5 nm zeolite, HZSM-5 nm zeolite, TiO2, γ-Al2O3, fumed SiO2 and SiO2-ball) were calcined at 400  C for 5 h to remove impurities such as H2O before impregnation, and then the pretreated supports were added to the metal nitrate solution. The mixture was kept at room temperature aging for 3 h, vacuum freeze-dried at
50  C overnight, and dried in air at 120  C for 5 h. Dried samples were calcined in a He DBD plasma at 540  C for 3 h. 1. Physicochemical properties of supports The data in Table 7.6 shows that the supports have different specific surface areas (Sg), average pore sizes and electrical properties. The electrical capacity (Cd) values of the supports are in the range of 27.77–36.91 pF, and the corresponding relative dielectric constants (εd) are in the range of 19.99–26.58. 2. Chemical states of supported metals The XRD patterns (Fig. 7.28) show that the supported Fe, Co, Ni, and Cu all underwent similar chemical state changes during reduction in the H2 plasma and plasma-catalytic NH3 decomposition, regardless of the supports used. For instance, over fumed SiO2, the as-prepared Fe, Co, Ni, and Cu existed as oxides (Fe2O3, Co3O4, NiO, and CuO), which were transformed to the metallic states (Fe, Co, Ni, and Cu) after reduction in H2 plasma. However, significant differences were observed when the metallic catalysts were used for plasma-catalytic NH3 decomposition, i.e., plasma-catalytic NH3 decomposition. The Ni and Cu catalysts remained in their metallic states, whereas the Co catalyst was partly transformed to the nitride, and the Fe catalyst was completely nitrided to form Fe3N and Fe4N. 3. Metal dispersion of catalysts TEM images of the metals supported on fumed SiO2 and Co supported on various supports are shown in Figs. 7.29 and 7.30, respectively. The average sizes of the Fe, Co, Ni, and Cu particles on the fumed SiO2 support were about 5, 3, 3–8, and 5 nm

Table 7.6 Physicochemical properties of supports used in NH3 decomposition through combination of supported catalysts and DBD plasma

Supports Fumed SiO2 SiO2-ball TS-1 nm zeolite γ-Al2O3 TS-1um zeolite HZSM5 zeolite NaZSM5 zeolite TiO2

Phase state Amorphous Amorphous Crystal Crystal Crystal Crystal

Specific surface area (Sg, m/g) 297.79 194.79 397.37 111.66 365.57 265.09

Average pore size (nm) – 7.97 0.55 6.97 0.54 0.55

Electric capacity (Cd, pF) 27.77 28.56 28.72 29.20 29.87 31.75

Relative dielectric constant (εd) 19.99 20.56 20.68 21.02 21.51 22.86

Crystal

273.08

0.54

33.33

24.00

–

36.91

26.58

Crystal (anatase)

14.22

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Fig. 7.28 XRD patterns of various metal catalysts supported on fumed SiO2: (a) Fe, (b) Co, (c) Ni, and (d) Cu. (Fe2O3 ICCD: 33-664, Fe ICCD: 6-696, Fe3N ICCD: 1-1236, Fe4N ICCD: 6-627, Co3O4 ICCD: 42-1467, Co ICCD: 15-806, NiO ICCD: 22-1189, Ni ICCD: 4-850, CuO ICCD: 41-254, Cu ICCD: 4-836) [88]

(Fig. 7.29), respectively, indicating high dispersion of the metals. Except NaZSM5 zeolite, highly dispersed Co particles were formed on the supports, i.e., fumed SiO2, SiO2-ball, TiO2, γ-Al2O3, TS-1 nm zeolite, TS-1 um zeolite, and HZSM5 nm zeolite (Fig. 7.30). The average Co particle size was less than 5 nm (mostly around 2–3 nm). In the case of NaZSM-5 zeolite, the average size of Co particles was found to be about 50 nm. 4. Metal
support interactions H2-TPR was used to investigate the metal
support interactions. The reduction of Co3O4 on the supports mainly occurred in the temperature range of 300–550  C (Fig. 7.31). The reduction temperatures increased in the order fumed SiO2 < SiO2ball < γ-Al2O3 < TS-1 nm zeolite < HZSM-5 nm zeolite < TS-1 um zeolite < NaZSM-5 nm zeolite < TiO2, indicating that the strength of interaction between Co particles and these supports increased in the same order. It was worth noting that the profiles of the Co3O4 reductions generally show two reduction peaks. The lower-temperature peak could be attributed to the reduction of Co3O4 to CoO,

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Fig. 7.29 TEM images of Fe, Co, Ni, and Cu catalysts supported on fumed SiO2 (reduced in H2 plasma) [88]

Fig. 7.30 TEM images of Co on various supports (reduced in H2 plasma) [88]

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Fig. 7.31 TPR profiles of Co catalysts on various supports [88]

and the higher-temperature peak could be assigned to the reduction of CoO to metallic Co [90–92]. In the case of the γ-Al2O3 support, a third reduction peak appeared in a much higher temperature region, i.e., 600–750  C, which could be assigned to reduction of Cox+ ions incorporated into the crystal lattice of the support [92, 93]. Such Cox+ ions have the strongest interactions with the support.

7.3.2.2

The NH3 Decomposition Results

The NH3 conversion increased significantly when a plasma was combined with the catalyst, regardless of which metals (Fig. 7.32) or supports (Fig. 7.33) were used [88]. The NH3 conversion achieved using “plasma + catalyst” mode was much higher than the sum of those obtained using plasma or a catalyst alone. For instance, over the Co/fumed SiO2 catalyst, the “plasma + catalyst” mode gave an NH3 conversion of 99.2%, whereas the sum of the NH3 conversions using only plasma or catalysis was only 26.0%. These results show a clear synergy between plasma and the catalysts. The synergy strongly depended on the types of metals and supports. It can be seen that the synergistic capability of the catalysts decreased in the order of Co > Ni > Fe > Cu, when fumed SiO2 was used as the support, and in the order of fumed SiO2  γ-Al2O3  SiO2-ball > TS-1 nm zeolite > HZSM-5 nm zeolite > TS-1 um zeolite > NaZSM-5 nm zeolite > TiO2 when Co metal was used as the active metal component.

7.3.2.3

Comparison of Metals

Generally, the metal dispersion has an important effect on the catalytic activity of a supported metal catalyst. However, Fig. 7.29 shows that the four metals (Fe, Co, Ni

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Fig. 7.32 Influence of metals on NH3 conversion in “plasma + catalyst,” “plasma” and “catalyst” modes [88]

Fig. 7.33 Influence of supports on NH3 conversion in “plasma + catalyst,” “plasma” and “catalyst” modes: A TiO2, B NaZSM5 nm zeolite, C TS-1 um zeolite, D HZSM5 nm zeolite, E TS-1 nm zeolite, F γ-Al2O3, G SiO2-ball, H fumed SiO2 [88]

and Cu) have similar particle size and similar loading. Figure 7.28 shows that the Fe and Co catalysts were mainly transformed into nitrides after NH3 decomposition reaction, while the Ni catalyst was probably transformed into surface nitrides. Compared with their metallic counterparts, the nitrided Fe, Co, and Ni catalysts all showed lower catalytic activities, as Pelka and co-workers reported [94]. Wang and co-workers demonstrated that the different activities in plasma-catalytic NH3 decomposition are closely related with the essential characteristic of the four elements (Fe, Co, Ni and Cu), i.e., the metal–N bond strength [88]. The relationship between the synergetic capability of the four supported catalysts and the metal–N strength will be discussed in Sect. 7.4.3.

7.3.2.4

Comparison of Supports

As mentioned in Sect. 7.3.2.2 (Fig. 7.33), the supports had a significant effect on the plasma
catalyst synergy in NH3 decomposition. The effect of the support on the synergy was investigated using a series of supported Co catalysts with similar Co loadings and similar distribution of Co particle sizes (Fig. 7.30). A comparison of the reaction results of NH3 decomposition and characterization results of the catalysts showed that the specific surface area, average pore size, phase state (crystalline or amorphous), and acidity of the support (Table 7.6) had no obvious effects on the

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Fig. 7.34 NH3 conversions on Co catalysts on various supports as a function of relative dielectric constants of supports in plasma + catalyst mode. (NH3 feed 40 mL/min, temperature 450  C, supported catalyst 0.88 g, discharge gap 3 mm, discharge frequency 12 kHz) [88]

synergy. The effect of support on the synergy was closely related to the metal
support interactions and the electrical properties of the support. Figure 7.31 shows that the order of the strengths of the interactions of Co with the supports was fumed SiO2 < SiO2-ball < γ-Al2O3 < TS-1 nm zeolite < HZSM5 nm zeolite < TS-1 um zeolite < NaZSM-5 nm zeolite < TiO2, which was consistent with the order of NH3 conversion in plasma-catalytic NH3 decomposition (Fig. 7.33). Clearly, weak interactions between Co and supports favored NH3 decomposition [88]. Figure 7.34 shows that the relative dielectric constant (εd) of the supports also correlated well with NH3 conversion of plasma-catalytic NH3 decomposition, i.e., the decomposition decreases significantly with increasing εd. In other words, a support with a small εd facilitated plasma-catalytic NH3 decomposition. To understand the influence of the relative dielectric constant of the support on the plasmacatalytic NH3 decomposition reaction, the NH3 decomposition was performed in the mode of “plasma + support” (Fig. 7.35). It can be seen that NH3 conversion still decreased significantly with increasing εd of the support. Taking NH3 conversion in “plasma” mode as a reference (NH3 conversion was 6%, shown by a dashed line), it can be found that fumed SiO2, γ-Al2O3, and SiO2-ball have weak but positive effects on NH3 conversion, whereas other supports such as HZSM-5 zeolite, NaZSM5 zeolite, and TiO2 have negative effects on NH3 conversion. The catalyst support usually plays an important role in heterogeneous catalysis, and the catalytic performances are generally related to the specific surface area, porosity, acidity and support
metal strong interactions. However, the electrical properties of supports have been hardly mentioned in heterogeneous catalysis. In the area of plasma catalysis, a few researchers have attempted to promote reactions by filling the plasma zone with high-dielectric-constant materials [95, 96]. The similarity in the sequences of εd
NH3 conversion relationships in Figs. 7.34 and 7.35 shows that the effect of εd on the NH3 conversion could be attributed to the influence of the support on the

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Fig. 7.35 NH3 conversions on different supports as a function of relative dielectric constants of supports in plasma + bare support mode. (NH3 feed 40 mL/min, temperature 450  C, bare support 0.88 g, discharge gap 3 mm, discharge frequency 12 kHz) [88]

discharge. In the plasma zone, the net electric field decreases because of accumulation of electrons on the solid surface, especially for supports with high εd.

7.4 7.4.1

Fundamental of the Synergetic Effect Between DBD Plasma and Cheap Metals Plasma Enhances the NH3 Adsorption Step on Metals

Wang and co-workers studied the fundamentals of the synergetic effects between DBD plasma and cheap metals on NH3 decomposition and found that plasma enhanced the NH3 adsorption step on metals [89]. Firstly, the NH3 plasma was diagnosed using OES. In the absence of Fe-based catalyst, the OES of NH3 plasma exhibited two electronically excited state species (Fig. 7.36). The continuum band from 564.3 to 568.1 nm (Schuster’s emission bands) is attributed to electronically excited state NH3 (NH3) [81]. The line at 337 nm (C3∏u ! B3∏g) is attributed to electronically excited state N2 (N2) [97]. In addition, three radical species, i.e., NH2 (390–830 nm) [81], NH (A3∏ ! X3S
, 336 nm) [98] and H (2p2p03/2 – 3d2D3/2, 656.3 nm), were detected in NH3 plasma as well. Among these species, NH3 was estimated to be the most abundant species since the signal intensity of NH3 species was the most intensive one. More importantly, the increase of the signal intensity of NH3 species as input power indicated that the amount of NH3 species correspondingly increased with the input power. In the presence of the Fe-based catalyst (Fig. 7.37), the OES of NH3 plasma also showed the existence of two electronically excited state species (NH3 and N2) and three radical species (NH2, NH and H); while the signal intensity of OES was much lower than that without the Fe-based catalyst, which might be caused by the

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Fig. 7.36 OES of NH3 plasma as a function of input power without a catalyst (1 atm, NH3 40 ml/ min, discharge length 4 cm, Fe-based catalyst 6 g, exposure time of OES 5 sec) [89]

Fig. 7.37 OES of NH3 plasma as a function of input power in the presence of an Fe-based catalyst (1 atm, NH3 40 ml/min, discharge length 4 cm, Fe-based catalyst 6 g, exposure time of OES 5 sec) [89]

shielding and absorption of the solid catalyst. More strangely, in the presence of an Fe-based catalyst, the signal intensity of the NH3 species first increased significantly with input power, but then decreased suddenly when the input power was higher than 26 W. This abnormal phenomenon indicates that packing an Fe-based catalyst in the plasma zone has effectively changed the composition of NH3 plasma under the conditions of high input power.

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Fig. 7.38 Relationship between NH3 OES signal and NH3 conversion as a function of input power in the presence of (a) and absence of (b) Fe-based catalyst (1 atm, NH3 40 ml/min, discharge length 4 cm, Fe-based catalyst 6 g, exposure time of OES 5 sec) [89]

Then, the NH3 OES signal was correlated with the NH3 conversion. As shown in Fig. 7.38a, in the presence of an Fe-based catalyst, the OES signal intensity of NH3 increased steadily and reached its maximum value at about 26 W input power. In this stage, the conversion of NH3 increased very slowly and stayed at a low level. Interestingly, when the input power reached more than 26 W, the OES signal intensity of NH3 decreased dramatically with the increase of input power, and the NH3 conversion soared up abruptly at the same time. As the conversion of NH3 reached its maximum value near 30 W, the OES signal intensity of NH3 fell down to its lowest value. This phenomenon suggested that, in plasma-catalytic NH3 decomposition, the abrupt increase of NH3 conversion might originate from the contribution of NH3. As shown in Fig. 7.38b, in the absence of the Fe-based catalyst, the OES signal intensity of NH3 first increased steadily with input power and then reached its maximum value at about 26 W. Meanwhile, the conversion of NH3 also increases slowly at a low level. However, in contrast to the case involving the presence of the Fe-based catalyst, a sharp decrease of the OES signal intensity of NH3 and quick increase of the NH3 conversion was not found at higher input power. Instead, both the OES signal intensity of NH3 and the conversion of NH3 remained almost constant. This distinct phenomenon indicates that, in the absence of Fe-based catalyst, the NH3 species had not been effectively consumed at high input power. Consequently, the conversion of NH3 was still very low even at high input power. The relationships of the OES signal intensity of NH3 and the conversion of NH3 in the presence and absence of Fe-based catalyst provide an idea that the NH3 species might have been efficiently consumed by the Fe-based catalyst and that the NH3 species as an active form of reactant showed the superiority in the adsorption activation step, as illustrated by Fig. 7.39. Wang and co-workers also designed some experiments to demonstrate this hypothesis. In addition, they found that, compared

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Fig. 7.39 Schematic diagram of adsorption activation of NH3 and ground-state NH3 on an Fe-based catalyst (E1, internal energy of NH3; Ea, adsorption activation energy of NH3; E2, adsorption activation energy of NH3) [89]

to the ground state NH3, NH3 species could obviously increase the adsorption capacity and adsorption strength on the Fe-based catalyst. The NH3 conversion was increased by nearly 40% due to the contribution of NH3 species under the condition of eliminating the influence of desorption step. Readers can find the experimental details in the reference [89].

7.4.2

Plasma Accelerates the Recombinative Desorption of Surface N Atoms

As mentioned in Sect. 7.1.2, the recombinative desorption of adsorbed N atoms is usually the rate controlling step when non-noble metal catalysts are used. Wang and co-workers demonstrated that plasma can accelerate the recombinative desorption of surface adsorbed N atoms [87]. Wang and co-workers studied the role of the DBD plasma in the synergy by monitoring the desorption process of Nad during ammonia decomposition in the absence or presence of DBD plasma (Fig. 7.40). The metallic Fe (pretreated in pure H2 flow) showed a very high capacity for producing H2 even without DBD plasma, indicating a high activity for ammonia decomposition, as shown in Fig. 7.40a. However, only a minor amount of N2 was detected and the H2 generation rate quickly decreased. In the presence of DBD plasma, however, a considerable amount of N2 was detected together with H2, and the H2 generation rate remained stable with time, as shown in Fig. 7.40b. These results show that metallic Fe is an active phase for NH3 decomposition but that the metallic Fe phase is easily poisoned by Nad atoms by forming less active iron nitrates. More importantly, it gives direct proof that plasma exhibits a powerful capacity to accelerate N2 desorption from the catalyst surface. How does the plasma accelerate N2 desorption from the surface of the catalyst? In-situ diagnosis with optical emission spectroscopy showed that active species such as NH3 and NH were present in the DBD plasma (Fig. 7.36). Then, a 15NH3 isotope tracing experiment was designed by Wang and co-workers to uncover the desorption process accelerated by plasma, as shown in Fig. 7.41.

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Fig. 7.40 Accelerating effect of DBD plasma on Nad removal from the metallic Fe catalyst in ammonia decomposition [87]. (a) Without DBD. (b) With DBD

Fig. 7.41 Scheme of the desorption of adsorbed N atoms

Fig. 7.42 Desorption process of 14N atoms from Fe214N was studied by 15 NH3 isotope tracing without DBD plasma [87]

A Fe214N catalyst was thermally prepared in an 14NH3 flow, and it was used as a Nad covered sample. Then, the 15NH3 decomposition reaction was conducted on the Fe214N catalyst in the absence or presence of DBD plasma, during which the

14

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Fig. 7.43 Desorption process of 14N atoms from Fe214N was studied by 15 NH3 isotope tracing with DBD plasma [87]

exhaust gas was detected by MS, as shown in Figs. 7.42 and 7.43, respectively [87]. In the absence of DBD plasma, 14N2 and then 14N15N appeared successively in the initial desorption stage (Fig. 7.42), indicating that the main way to desorb N2 was the self-recombinative desorption of 14Nad. Only after part of the 14Nad desorbed from the catalyst surface, the 15NH3 in the gas phase could adsorb on the catalyst and then produce 14N15N and 15N2. In the presence of the DBD plasma (Fig. 7.43), however, the initial desorption product was 14N15N but not 14N2, and the 14N15N could mostly be produced through the interaction of 14Nad and gas-phase 15N active species (15NH3, 15NH) by an Eley-Rideal mechanism. In addition, comparison of the desorption spectra of 15N2 in the presence or absence of DBD plasma in the later desorption stage showed that the main desorption of 15N2 appeared after the desorption of 14N15N and 14N2 in the presence of DBD plasma (Fig. 7.43), but appeared close together with 14N15N and 14N2 in the absence of DBD plasma (Fig. 7.42). These results indicate that most 15N species in the gas-phase were directly consumed by reaction with the 14Nad to form 14N15N (Eley-Rideal mechanism) in the presence of plasma. The results also support the idea that the reaction of the active species (NH3, NH) with surface active Nad may be the main approach for the desorption of Nad accelerated by the plasma technique. Zhang and co-workers also reported an interaction of NH and NH2 radicals with non-noble metal catalyst surfaces [99].

7.4.3

Dependence of Synergetic Effect on Metal–N Bond Strength

As shown in Sects. 7.3.2.2 and 7.3.2.3, when fumed SiO2 was used as the support, the synergistic capability of the catalysts decreased in the order of Co > Ni > Fe > Cu,

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Fig. 7.44 IR spectra for NH3 adsorption on Fe, Co, Ni, and Cu catalysts supported on fumed SiO2. (Samples were treated by in situ H2 reduction at 500  C, and NH3 adsorption was conducted at 350 and 450  C, respectively, followed by outgassing at 200  C) [88]

which may be closely related to the metal–N bond strength. Wang and co-workers studied the metal–N bond strength of Co, Ni, Fe and Cu, using adsorption and desorption experiments [88]. The adsorption of NH3 on the Fe, Co, Ni, and Cu catalysts was studied using FTIR. There was a significant number of adsorbed NH2 (NH2,ad) species on the catalyst surfaces, but no NHad species were detected (Fig. 7.44). The attribution of these species in FTIR can be found in references [100, 101]. The amount of NH2,ad varied depending on the metals and the temperature. When the adsorption temperature was increased from 350 to 450  C, the NH2,ad species on the Cu catalyst disappeared, but the amount of such species on the Fe catalyst was almost unchanged. These results indicate that dissociative adsorption of NH3 occurred on these catalysts, and different metals had different bonding abilities toward NH2,ad species. In other words, the Cu
NH2,ad bond was the weakest one, while the Fe
NH2,ad bond was the strongest one. During NH3 decomposition on the supported Fe, Co and Ni catalysts in the absence of DBD plasma, an intense H2 signal was detected almost immediately after the injection of NH3 (Fig. 7.45). However, only a weak N2 signal was observed for these catalysts. The H2 signal attenuated sharply with time and then leveled off. These results show that H2 desorption is easy, but N2 desorption from these metals is difficult in the absence of a plasma. The majority of N atoms were adsorbed by Fe, Co, and Ni catalysts through forming strong M
N bonds. The H2 and N2 signals for the Cu catalyst were much weaker than those for the Fe, Co, and Ni catalysts, which could be caused by poor activity of the Cu catalyst in NH3 decomposition. The relative M
N bond strengths (M ¼ Fe, Co, Ni, and Cu) were investigated using TPD. Figure 7.46 shows that considerable amounts of N2 were released from the Fe, Co, and Ni catalysts, and it may be the reason why only small amounts of N2 were detected by MS and the H2 signal intensities decreased with time during NH3 decomposition over the catalysts in the absence of a plasma (Fig. 7.45). As shown in Fig. 7.46, the temperature needed for N2 desorption from the catalysts’ surface

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Fig. 7.45 On-line MS analysis of the initial stage of NH3 decomposition over Fe, Co, Ni, and Cu catalysts supported on fumed SiO2 without plasma. (0.95 g of metal catalyst was prepared in a H2 flow for 3 h at 500  C and cooled to 450  C in a He flow to remove H2; the flow was switched to 40 mL/min NH3 for the reaction at 450  C) [88]

Fig. 7.46 TPD profiles of supported Fe, Co, Ni, and Cu catalysts obtained from the same experiment as in Fig. 7.45. (After the reaction, these catalysts were cooled in situ to room temperature in an NH3 flow) [88]

increased significantly from Cu to Ni, Co, and Fe, indicating that the strength of the M
N bond increased from Cu to Ni, Co and Fe, i.e., the Fe
N bond was the strongest, and the Cu
N bond was the weakest, which is consistent with the results of FTIR (Fig. 7.44). The M
N bond strength is one of the key factors in NH3 decomposition over cheap metal catalysts. Metals with moderate M
N bond strengths (Co
N) showed higher activities (Figs. 7.32 and 7.46). If the M
N bond was too weak, dissociative adsorption of NH3 on the metal is difficult, and desorption of the NH2,ad intermediate species takes place easier than further dehydrogenation, resulting in low catalytic activity. This was the case for the supported Cu catalysts (Fig. 7.44). Boisen and co-workers reported that the activity of Cu in catalytic NH3 decomposition is lower than those of other metal catalysts [102]. However, if the M
N bond was too strong, dissociative adsorption of NH3 on the metal occurred easily, but desorption of the

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Fig. 7.47 On-line MS analysis of the initial stage of NH3 decomposition over Fe, Co, Ni, and Cu catalysts in plasma + catalyst mode. (Fumed SiO2 was used as the support; 0.95 g of metal catalyst were prepared in H2 flow for 3 h at 500  C, followed by cooling to 250  C in He flow to remove H2 and then switching to 40 mL/min NH3 flow for reaction under AC supply power about 28 W, and the final temperature remained around 450  C) [88]

bonded N atoms from the catalyst was difficult, and therefore highly active metal sites were difficult to recover. This was the case for the Fe catalysts (Figs. 7.45 and 7.46). The reason for the different synergies of Fe, Co, Ni, and Cu catalysts with plasma, as shown in Fig. 7.32, can be mainly attributed to their different M
N bond strengths. It should be pointed out that although Co has a moderate M
N bond strength, NH3 decomposition over the Co catalyst alone gave very low conversion (~20%), as shown in Fig. 7.32. The recombinative desorption of the bonded N atoms is still the rate-limiting step. However, when NH3 decomposition was performed in “plasma + Co catalyst” mode, NH3 conversion reached almost 100%. Comprehensively, the role of the plasma in the synergy is to enhance the NH3 adsorption step on metals and accelerate the recombinative desorption of bonded N atoms. Figure 7.47 gives more direct proof to demonstrate the role of the plasma in the plasma
catalyst synergy for NH3 decomposition. Clearly, this acceleration effect of the plasma works not only in the case of Co catalysts, but also for Fe and Ni catalysts.

7.5

Perspectives of Ammonia as a Carrier for Hydrogen Energy Circulation

Ammonia is a non-carbonyl hydrogen source, and NH3 as a source of hydrogen does not emit greenhouse gases. The hydrogen for synthesis of NH3, however, is mostly from carbonaceous fossil raw materials. That is, in conventional NH3 synthesis a large amount of greenhouse gas is emitted. Therefore, the synthesis process of ammonia in industry when based on fossil raw materials is not a green route. As everyone knows, water is an inexhaustible source of hydrogen. The only by-product of hydrogen production from water is oxygen, and the hydrogen in the fuel cell will

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Fig. 7.48 Potential route of a hydrogen energy circle using NH3 as a carrier

consume the same amount of oxygen. As a result, water is undoubtedly the greenest source of hydrogen. In the near future, humans may be able to adopt the following ways to use hydrogen, as shown in Fig. 7.48. (i) Continuous hydrogen production using H2O as the initial hydrogen source and using solar energy, wind energy, tidal energy, geothermal energy and some other green and sustainable energy as the initial energy. Hydrogen production sites can be selected in areas where water is abundant and green and sustainable energy can be obtained easily, such as the coast. Decomposition of water to hydrogen can be achieved using photocatalysis, electrolysis, etc. (ii) In-site synthesis of NH3 using hydrogen produced from H2O and nitrogen obtained by air separation. The current industrial technology and process for the synthesis of NH3 from hydrogen and nitrogen can be used. In order to avoid the emission of greenhouse gases, all the energy consumption in NH3 synthesis and air separation should fully use solar and wind energy, tidal energy, water and other green and sustainable energy. (iii) NH3 liquefaction, storage and decomposition for hydrogen production. NH3 can be liquefied at room temperature under 0.8 MPa pressure. Currently, the storage and transportation of liquid NH3 is also a mature technology. NH3 decomposition can be carried out using conventional heterogeneous catalytic method or the plasma catalytic method. (iv) H2/O2 fuel cell reaction. PEMFC, an industry-proven process, produces a power to drive vehicles. In general, this hydrogen cycle route includes two groups of reversible reactions (water decomposition and synthesis, ammonia synthesis and decomposition), circulation of N2 and O2 in the atmosphere and circulation of hydrogen on earth. This proposal aims to provide a potential route to use hydrogen energy without involving greenhouse gas emission, which is always the purpose of developing hydrogen energy.

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Acknowledgment We acknowledge financial support from the China Postdoctoral Science Foundation [2016T90217 and 2015M580220], the National Natural Science Foundation of China [20473016, 20673018, 21503032], the Fundamental Research Funds for the Central Universities [DUT18JC42], and the China Petroleum Science and Technology Innovation Fund Research Project [2018D-5007-0501]. We also acknowledge Dr. Yue Zhao, Prof. Weimin Gong and Prof. Jialiang Zhang for their contribution on the research of NH3 decomposition.

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Chapter 8

Plasma-Catalytic Conversion of Methane Tomohiro Nozaki, Seigo Kameshima, Zunrong Sheng, Keishiro Tamura, and Takumi Yamazaki

8.1

Introduction

Methane is one of the cleanest and most abundant primary energy, carbon, and hydrogen sources among the available hydrocarbon resources. The growing demand for shale gas accelerates the need for methane as a promising future energy source. In addition to its direct use at thermal power plants, steam methane reforming (SMR) has been used for various chemical industries as well as being a hydrogen source for ammonia synthesis and fuel cell applications. SMR has a long and proven history and its related technology is well developed. Nevertheless, further improvement of energy and material conversion efficiencies is demanded. Moreover, an urgent demand for CO2 free hydrogen inevitably requires innovative new technologies for the sustainable use of methane. More recently, the power-to-gas concept has been highlighted where water electrolysis driven by renewable electricity is utilized. Renewable hydrogen is further combined with the catalytic conversion of CO2 to synthetic natural gas (SNG) [1]. Renewable H2 and SNG can be stored and distributed through the existing gas grid for efficient use of renewable energy with reduced CO2 emissions. Based on this similar concept, we investigated CH4 reforming using nonthermal plasma-enhanced catalytic reaction [2–4]. Analogous to electrolysis of water splitting, renewable electricity is converted into chemical energy via nonthermal plasmaassisted endothermic reactions. Syngas is then upgraded to H2-rich gas or SNG for power-to-gas applications. Alternatively, liquid fuels such as gasoline and methanol are synthesized through well-established C1 chemistry using plasma-generated synthetic gas. Carbon-containing liquid fuels are particularly important because their energy density is 10–100 times greater than that of secondary batteries, meaning the transport and storage capability of renewable energy are greatly T. Nozaki (*) · S. Kameshima · Z. Sheng · K. Tamura · T. Yamazaki Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, Japan e-mail: [email protected] © Springer Nature Switzerland AG 2019 X. Tu et al. (eds.), Plasma Catalysis, Springer Series on Atomic, Optical, and Plasma Physics 106, https://doi.org/10.1007/978-3-030-05189-1_8

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improved, while CO2 capture and utilization are also strengthened simultaneously. Currently, electrochemical reactions are primarily studied for renewable-to-chemical energy conversions. Nonthermal plasma can provide additional energy and material conversion pathways, contributing to an extended carbon recycling network and material flexibility [4–10]. Over the next decade, a sufficient amount of renewable electricity will be available through photovoltaics and wind turbines, where energy conversion efficiency and power distribution technology are rapidly growing worldwide. Nonthermal plasma features compactness, a flexible reactor design, low-temperature operation, and load followability: which are compatible with renewable energy sources and the various chemical reactors developed so far. Renewable energy-driven nonthermal plasma catalysis provides a viable solution for efficient energy and material conversion, storage, and transportation with reduced CO2 emissions. This chapter focuses particularly on dielectric barrier discharge (DBD) and catalyst combination for methane conversion to synthesis gas (H2, CO). First, the plasma catalysis of methane is reviewed in terms of the endothermic and exothermic nature of the reaction systems where the role of nonthermal plasma is essentially different. After that, the dry reforming of methane (CH4 + CO2 ! 2CO + 2H2) in DBD/catalyst hybrid reaction is highlighted based on our recent study. The basics of heterogeneous reaction, known as a Langmuir-Hinshelwood mechanism, are overviewed for the better understanding of nonthermal plasma and surface interaction as well as plasma-enhanced heterogeneous reactions, which is known as plasma catalysis. Pulsed reaction spectrometry is introduced as a powerful diagnostic approach of heterogeneous reaction kinetics under the influence of nonthermal plasma. Moreover, the nonthermal plasma-induced synergistic effect and energy efficiency are discussed in order to move toward a deeper understanding of plasma catalysis in methane reforming. Finally, concluding remarks and the future outlook are presented.

8.2

Plasma Catalysis for CH4 Reforming

In general, CH4 conversion indicates syngas (H2 and CO) production via steam reforming (8.R1), dry reforming (8.R2), partial oxidation (8.R3), and the combination of 8.R1, 8.R2 and 8.R3 known as autothermal reforming. Syngas is converted into various synthetic chemicals by well-established C1 chemistry [11]. In particular, Fischer-Tropsch hydrocarbon fuels have attracted keen attention in view of their high energy storability and transportability in the form of liquid at ambient conditions: CH4 þ H2 O ! 3H2 þ CO



ΔH ¼ 206 kJ=mol



ΔG ¼ 142 kJ=mol

ð8:R1Þ

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CH4 þ CO2 ! 2H2 þ 2CO ΔH ¼ 247 kJ=mol ΔG ¼ 170 kJ=mol 



CH4 þ 0:5O2 ! 2H2 þ COΔH ¼ 36 kJ=molΔG ¼ 87 kJ=mol 

ð8:R2Þ ð8:R3Þ



Here, ΔH and ΔG , respectively, represent the standard enthalpy and Gibbs free energy changes of the reaction. Hereafter, H2O is considered in the vapor phase. If H2 is demanded for fuel cell application, the water gas shift (WGS) reaction (8.R4) is induced to enrich H2 in the upgraded gas stream: CO þ H2 O ! H2 þ CO2



ΔH ¼ 41 kJ=mol



ΔG ¼ 29 kJ=mol

ð8:R4Þ

The role of nonthermal plasma can be uniquely distinguished into two categories depending on whether it is combined with an exothermic or endothermic system, which is discussed further in the following sections.

8.2.1

Exothermic System

8.2.1.1

CH4 Partial Oxidation in Gliding Arc Discharge

One of the important roles of nonthermal plasma is to generate reactive species almost independently of the ambient temperature. Plasma-generated reactive species are used to initiate chemical reactions at much lower temperatures than conventional thermochemical reactions. Such peculiarity is particularly important in non-catalytic homogeneous partial oxidation of hydrocarbons [12] and plasma-assisted combustion [13]. The main driving force of an exothermic reaction is the heat released by partial oxidation of the initial feed. Although generation of nonthermal plasma consumes electrical energy, plasma itself is not necessarily an energy source in the exothermic system; therefore, the energy consumption of plasma-assisted partial oxidation is inevitably low when compared to an endothermic system. Reaction temperature, fuel/oxygen ratio, and plasma power can be varied to a large extent as long as plasma-generated reactive species induce a spontaneous chain reaction to sustain partial oxidation. Because plasma can lower the reaction temperature (seemingly, lowering the apparent activation energy) and create new reaction pathways, plasma is considered to possess a catalytic effect which is called plasma catalysis. However, the definition of plasma catalysis is not always clear. Because the generation of any type of plasma source is accompanied by heat generation. The individual contribution of radical injections, local heating, or a combination of these, it is not always well understood. One of the most successful applications is the gliding arc discharge for the partial oxidation of hydrocarbon fuels. Figure 8.1 shows gliding arc discharges in various electrode configurations. A diverging two-blade electrode reactor is commonly used to generate relatively intense discharge channels (Fig. 8.1a) [14]. It is characterized as a two-dimensional transient arc plasma because discharge channels are generated

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Fig. 8.1 Gliding arc type discharges in various configurations: (a) Diverging two-blade type (2D) [14], (b) Reverse vortex flow “tornado” type (3D) [14], (c) Rotating arc type [16]

in the planar geometry, and thus contact between discharge channels and the gas stream may be insufficient. In order to improve the interaction between the discharge channels and incoming gas flow, a three-dimensional configuration known as a reverse vortex flow, or tornado gliding arc discharge, was developed (Fig. 8.1b) [14]. The heat generated by the transient high-temperature arc column, as well as excited species, plays a key role. The principles of thermal and nonthermal characteristics of gliding arc discharge are investigated in detail by Fridman and co-workers [15]. A similar principle, but different electrode configuration, is named rotating arc discharge (Fig. 8.1c) [16]. Transient filamentary arc channels are literary rotating between diverging coaxial electrodes, improving the mixing and ionization of the reactant flow. Thermal and nonthermal characteristics of rotating arc discharges and their applications in fuel reforming are presented by Song and co-workers [17]. Gliding arc discharges have shown some of the best reforming performances for syngas production [7]. However, there are several challenges to overcome before widespread use in industry. First, relatively high temperatures under fuel-rich

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conditions produce coke and/or tar [18]. Second, the use of air as an oxidizer dilutes the output gas with N2 by up to 50%, producing low calorific syngas [19]; alternatively, N2 should be separated by post-plasma processes. The use of pure O2 accompanies an additional energy penalty and a bulky air separation unit which is not compatible with a compact and low-cost reforming system. Pure O2 from water electrolysis, obtainable from a power-to-gas system (water electrolysis), is an interesting option in future applications. Finally, full oxidation of fuels is likely to occur in the fuel/O2 system. Syngas can be generated by the full oxidation of fuels, followed by endothermic steam/dry reforming as shown in Fig. 8.2. The principle is similar to a multistep reforming as depicted in the in-direct route: drastic improvement of energy efficiency may be hard to achieve unless CH4 combustion is excluded.

8.2.1.2

Direct Conversion of CH4 to CH3OH in Microplasma Reactor

An alternative approach for gliding arc partial oxidation of CH4 is a low-temperature and single-step CH4 partial oxidation to methanol as indicated in direct route (Fig. 8.2). Direct conversion of methane to methanol can greatly reduce capital and operating costs of high-temperature, energy-intensive, multistep processes for syngas production [20, 21]: CH4 þ 0:5O2 ! CH3 OH



liq

ΔH ¼ 163 kJ=mol



ΔG ¼ 116 kJ=mol ð8:R5Þ

Although tremendous effort has been made on the direct CH4 conversion to oxygenates in a homogeneous gas phase reaction and over heterogeneous catalysts [22– 25], the yield for desired products was below the economic value [26]. More recently, atmospheric pressure nonthermal plasma is looked at as a viable synthesis

Fig. 8.2 Energy and material conversion pathways in CH4 reforming: direct (I.-III.) and in-direct (I.-II.-III.) routes

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method because high-energy electron impacts initiate CH4 partial oxidation at low temperature, enabling a single-step CH4 conversion into various oxygenates. Nevertheless, the one-pass yield for useful oxygenates was unsatisfactory [27– 29]. Performing nonthermal discharge in a microreactor brings unconventional thermochemical conditions to materials processing, enabling better control over process parameters to selectively synthesize desirable products [30, 31]. DBD generated in a microreactor selectively produced methanol (CH3OH), formaldehyde (HCHO), and formic acid (HCOOH) with a one-pass yield of more than 10% [32]. A schematic diagram of the microplasma reactor is presented in Fig. 8.3. Dielectric barrier discharge is generated in a thin glass tube (inner diameter, 1.5 mm, and Pulsed power supply MFC Digital water pump

O2

MFC CH4

Water bath (ca.10 °C)

Thermostat GND

Cold trap ca. 10 °C

GC, QMS GC-MS

Fig. 8.3 A schematic diagram of the two-phase flow microplasma reactor and discharge images: (a–b) without water injection, (c–e) with water injection. The emission from discharge looks like (b) and (e) to the naked eye. Magnified images (c) and (d) show the water film on the wall. Conditions: 30 W and CH4 ¼ O2 ¼ 20 cm3/min

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length, 50 mm) equipped with an internal wire electrode (diameter, 0.2 mm). Gas breaks down with a nanosecond duration, and average gas temperature only slightly increases [33–35]. Meanwhile, high-energy electrons activate methane and oxygen, initiating CH4 partial oxidation at room temperature; therefore, a stoichiometric flammable CH4 and O2 mixture (8.R5) was processed in the microplasma reactor without detonation. Moreover, distilled water was injected simultaneously from the top of the reactor, creating a water film on the inner wall of the tube. The microreactor configuration enhances the removal of heat generated by CH4 partial oxidation. Therefore, ignition and propagation of flames, or full combustion, in CH4/ O2 mixture are avoided. Moreover, the low-temperature operation enhances the condensation of liquid components onto the water film, realizing product separation from the oxygen containing reactive plasma. Product separation is important because organic oxygenates are much more reactive than CH4 and readily decompose during oxidative methane activation. Figure 8.3a, b shows images of DBD without water injection. Although DBD consists of a number of filamentary discharges, the timeand space-averaged image is fairly uniform. In contrast, DBD is no longer uniform as a result of complex interaction with the water film (Fig. 8.3c–e). A high electric field creates small bumps on the water film due to the electrohydrodynamic interaction. The electric field is concentrated at these points, creating constricted and intense discharge channels. The total selectivity of liquid components is shown in Fig. 8.4 with respect to CH4 conversion [36]. The results are compared with ten from cited literature that used thermochemical methods [24, 25, 37–44]. This literature was selected mostly from the excellent review article given by Rasmussen and Glarborg [45]. A similar attempt was made by Casey and co-workers [26], comparing selected cited literature in terms of CH3OH selectivity vs. CH4 conversion. Although tremendous effort has been made toward direct CH4 conversion to CH3OH, there is no significant improvement between 1994 [26] and 2008 [45]. In the conventional thermochemical approach, relatively high-temperature thermal energy must be added to initiate methane partial oxidation, while a small amount of oxygen, typically 10% at most, was introduced so that successive oxidation of methanol is suppressed. As a result, CH4 conversion beyond 10% was scarcely achieved. Although CH4 conversion increased by increasing either the oxygen content or reaction temperature, selectivity for useful oxygenates dropped sharply. There is a clear trade-off relationship between CH3OH selectivity and CH4 conversion. Rasmussen and Glarborg studied CH4 partial oxidation numerically, assuming fuel-rich, high-pressure (30–100 bar), and relatively low-temperature conditions (550–800 K). They concluded the highest one-pass methanol yield of 4.2% would be feasible at optimum conditions of 97.4 bar, 643.3 K, and the initial CH4/O2 ratio of 23.6 [45]. Theoretical prediction does a good job explaining experimental results reported so far, implying that the thermochemical method may not be a promising approach for direct conversion of methane to methanol at a high yield. In contrast, the total liquid selectivity (●) in the two-phase flow microplasma reactor decreases moderately with respect to CH4

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Fig. 8.4 Total liquid selectivity (● and ■) vs. CH4 conversion. Conditions, O2/CH4, 0.5 and 1.0; reaction time, 130–530 ms. The three curves represent constant yield lines. Data is compared with cited literature values [24, 25, 37–44]

conversion. Although liquid selectivity was below 60%, a one-pass liquid yield with ca. 20% was obtainable. Assuming post DME (dimethyl ether, CH3OCH3) synthesis with the syngas obtained as by-product, the overall liquid selectivity could be enhanced up to 80% (■). Atomic oxygen is considered as a key species to initiate CH4 partial oxidation at low temperature [46]: CH4 þ O ! CH3 þ OH



ΔH ¼ 11:7 kJ=mol

Ea ¼ 31:8 kJ=mol

ð8:R6Þ

ΔH expresses standard enthalpy change and Ea is the activation energy [47]. O2 dissociation by electron impact produces one ground-state atomic oxygen atom (O (3P)) and an electrically excited one (O(1D)) [48]:

 O2 þ e ! O 3 P þ O 1 D þ e

 O2 þ e ! O 3 P þ O 3 P þ e

ð8:R7Þ ð8:R8Þ

However, based on the numerical simulation, neither O(3P) nor O(1D) abstract a H-atom from the strong C-H bond of CH4 below 300  C [49]. Presumably, 8.R6 is an endothermic reaction and the thermal energy (high-temperature environment) is needed to compensate the reaction enthalpy. Atomic oxygen can be a key species at elevated temperatures, but the results more closely resemble the total oxidation of CH4, which contradicts low-temperature direct CH4 conversion to CH3OH. On the

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other hand, OH is apparently a desired oxidative agent because CH dissociation proceeds via an exothermic reaction with a much smaller activation energy (8.R9) than 8.R6 [47]: CH4 þ OH ! CH3 þ H2 O



ΔH ¼ 60:9 kJ=mol

Ea ¼ 10:2 kJ=mol ð8:R9Þ

CH4 þ H ! CH3 þ H2 CH4 þ M ! CH3 þ H þ M



ΔH ¼ 3:35 kJ=mol 

ΔH ¼ 440 kJ=mol

Ea ¼ 36:6 kJ=mol

ð8:R10Þ

Ea ¼ 446 kJ=mol ð8:R11Þ

C-H dissociation by atomic hydrogen (8.R10) is also possible when ΔH and Ea are of the same levels as 8.R6. However, as the recombination of H and CH3 (backward 8.R11) is preferable to 8.R10, CH4 conversion would decrease which is against the expectation [49, 50]. CH4 pyrolysis (8.R11) at low temperature is negligible because of the large activation energy and endothermic enthalpy. The electron impact CH4 dissociation is the key initiation pathway at room temperature (8.R12), and CH3 is oxidized consecutively toward CH3OH, HCHO, and HCOOH [49]. As predicted by the model reaction scheme (8.R12, 8.R13, 8.R14 and 8.R15), a large amount of methyl hydroperoxide (CH3OOH) was detected in the plasma-treated liquid sample as the key intermediate species [51], implying that total liquid selectivity shown in Fig. 8.4 should further increase with appropriate posttreatment of the plasma-treated liquid. As CH4 conversion increases, partial oxidation of H2 occurs preferentially to CH4 oxidation, producing oxidative species such as H2O2, OH, and HOO that further accelerate H2 partial oxidation [50]. Consequently, H2 selectivity decreased markedly and CH4 oxidation was decelerated: H2 is clearly detrimental for efficient conversion of CH4 to CH3OH. Although nonthermal plasma creates unconventional reaction pathways at low temperature, it is hard to overcome trade-off relationship between CH4 conversion and product selectivity:

8.2.2

CH4 þ e ! CH3 þ H þ e

ð8:R12Þ

CH3 þ O2 ! CH3 OO

ð8:R13Þ

CH3 OO þ CH4 ! CH3 OOH þ CH3

ð8:R14Þ

CH3 OOH ! CH3 OH, HCHO, HCOOH

ð8:R15Þ

Endothermic System

Methane and CO2/H2O reforming is an endothermic reaction requiring hightemperature (>800  C) thermal energy, which is supplied by the combustion of

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initial feed. It is truly detrimental because it directly leads to energy loss and releases CO2 as well as NOx: if the catalytic effect of nonthermal plasma lowers the reaction temperature and eliminates combustion of initial feed, it is beneficial for the greener use of a natural gas resource. Low-temperature waste heat can be used as an external heat source; however, unlike exothermic systems, the energy consumed by nonthermal plasma should cover a part of energy necessary for the endothermic reaction: nonthermal plasma is not only a radical source (reaction promotor) but also an energy source. It is noteworthy to mention that, in the case of a power-to-gas concept, energy consumption by nonthermal plasma is not necessarily low because excess renewable electricity needs to be converted to the chemical energy of syngas. The key issue is to pursue an appropriate combination of low-temperature thermal energy and electrical energy (energy consumed by plasma) in order to maximize energy and the material conversion efficiency.

8.2.2.1

Steam CH4 Reforming in DBD/Catalyst Hybrid

A comprehensive analysis of plasma catalysis of CH4 is presented by T Nozaki et al. [2]. Briefly, an energy efficiency of 10% was estimated for CH4 conversion to C2 hydrocarbons (mostly C2H6) by a simplified one-dimensional numerical analysis of the streamer-type discharge produced in pure CH4 at ambient conditions (300 K and 101.3 kPa). It was also studied by the experiments, showing an energy efficiency of less than 1% was feasible: more than 90% of energy fed into the DBD was converted into heat and removed by water-cooled electrodes [34]. Another 10% was used for gas heating. In contrast, the energy efficiency of CH4 conversion to syngas reached 50–80% when DBD was combined with a Ni/Al2O3 catalyst [52, 53]. The importance of the vibrational excitations of CH4 and their interactions with heterogeneous catalysts was pointed out in the nonthermal plasma catalysis of steam methane reforming [54, 55]. The effect of radical injections on the reaction enhancement was kinetically analyzed by the Arrhenius plot method. The forward methane reaction rate based on the overall steam methane reforming using power-law kinetics is expressed as follows: r¼

d½CH4  ¼ k½CH4 α ½H2 Oβ ¼ k½CH4 ð0:851:4Þ ½H2 Oð0:80Þ dt   E k ¼ A  exp  RT

ð8:1Þ ð8:2Þ

Here, r denotes the forward methane reaction rate and k is the forward methane conversion rate constant for the overall SMR reaction. The reaction order for overall SMR on a nickel catalyst is α ¼ 0.85~1.4 in CH4 and β ¼ 0.8~0 in H2O [56– 59]. The phenomenological understanding of those values is that the ratedetermining step is the activation of methane and the overall forward reaction rate

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Fig. 8.5 An Arrhenius plot for the forward methane reaction rate constant. ○, ●, GHSV ¼ 18,000 h1, S/C ¼ 1; □, ■, GHSV ¼ 18,000 h1, S/C ¼ 3; Δ, ~, GHSV ¼ 10,800 h1, S/C ¼ 1, P ¼ 101.3 kPa

is less dependent on H2O [60]. Assume the overall rate constant is expressed in the Arrhenius form (Eq. 8.2), the results are shown in Fig. 8.5. An earlier study [61] presented detailed procedures to evaluate the reaction order (α and β), pre-exponential factor (A), and overall activation energy (E) under the influence of DBD. The convex characteristics of the Arrhenius plot imply that overall SMR was in the reaction-limited regime when the catalyst temperature was lower than 460  C. The diffusion-limited regime was clearly distinguished at this temperature. An analogous curve is obtained in the hybrid reaction where the reaction regime is separated at 420  C. The overall activation energy in the reaction-limited regime is approximately 100 kJ/mol, which agrees well with the reported value [60], indicating that dissociative chemisorption of CH4 over a Ni catalyst is the rate-determining step independent of DBD hybridization. The same trend is observed in the diffusionlimited regime, where only the pre-exponential factor was enhanced sevenfold, while the apparent activation energy was not influenced by the DBD. Figure 8.5 derives important conclusions and the hypothesis is as follows: 1. The overall activation energy was not influenced by DBD: the rate-determining step, i.e., C-H dissociation over catalyst, seems to be essentially unchanged by nonthermal plasma. The contribution of plasma-activated CH4 is not clearly identified by the macroscopic observation. 2. Plasma-generated reactive species, such as OH, do not induce an alternative CH4 activation pathway in the gas phase as the overall activation energy was unchanged. Therefore, the reaction mechanism of plasma-mediated catalysis could be explained by extending a Langmuir-Hinshelwood surface reaction mechanism.

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3. The reaction order for CH4 (α) was only slightly increased by DBD, while for H2O (β), it was more than doubled [61]. 4. The pre-exponential factor was clearly increased by DBD. One possible explanation is that plasma-activated H2O removes adsorbed carbon species which regenerates active sites for subsequent CH4 adsorption; thus the number of apparent active sites increases, but the CH4 activation rate over individual sites would be kept unchanged. 8.2.2.2

Dry CH4 Reforming in a DBD/Catalyst Hybrid

Dry methane reforming (DMR) has drawn keen attention as a viable CO2 utilization technology as it may have one of the greatest commercial potentials [62, 63]. Various catalysts have been developed and reaction kinetics are well understood [64, 65]. However, a commercial installation of DMR is still difficult because the coke formation and catalyst deactivation problems have yet to be fully solved. An efficient supply of high-temperature heat is necessary; however the heat transport property of a fixed bed reactor is generally poor. Therefore, an excessively hightemperature operation is required which increases the energy penalty and deteriorates materials used for the reactor and catalysts. An increasing demand of DMR is therefore pursuing a new technology, and potential use of thermal and nonthermal plasma is highlighted. Dielectric barrier discharge is the most successful atmospheric pressure nonthermal plasma sources in industrial applications [66]. Fundamental physics and chemistry in DBD are well understood in relation to ozone synthesis. More recently, DBD and its combination with heterogeneous catalysts have been investigated for plasma-assisted dry methane reforming [3, 67–75]. Hereafter, plasma-assisted dry methane reforming over Ni/Al2O3 catalysts is focused based on our recent study [3, 74, 75].

8.3 8.3.1

Dry Methane Reforming over Heterogeneous Catalyst Thermodynamics

Dry methane reforming is expressed by 8.R2. In addition, solid carbon (C) and H2O are produced as undesired by-products: ν1 CH4 þ ν2 CO2 ¼ ν3 H2 þ ν4 CO þ ν5 C þ ν6 H2 O

ð8:R16Þ

νi expresses the stoichiometric coefficient for each component. Figure 8.6a, b, respectively, illustrates the equilibrium molar fraction for the six species (8.R16) with a stoichiometric mixture of CH4/CO2 ¼ 1 at constant pressures of 100 kPa and 5 kPa. The CH4 and CO2 conversions are relatively high even below 500  C; however, the selectivity for C and H2O is much greater than that of syngas

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Fig. 8.6 Equilibrium composite for dry methane reforming at CH4/CO2 ¼ 1:1. (a) Total pressure 100 kPa and (b) 5 kPa

(H2 and CO), for C especially where selectivity is nearly 100% below 500  C at a total pressure of 100 kPa. This is explained by the Boudouard equilibrium (8.R17); carbon formation is thermodynamically favored at low temperature: CO þ CO ¼ C þ CO2



ΔH ¼ 172 kJ=mol



ΔG ¼ 120 kJ=mol ð8:R17Þ

One of the major roles of nonthermal plasma is to lower the reaction temperature. Therefore, it is critically important to elucidate the reaction mechanism involving H2O and C under the influence of nonthermal plasma. As shown in Fig. 8.6b, the production of syngas is shifted toward low temperatures at reduced pressure. Correspondingly, formation of H2O and C shifts to the lower-temperature regime. Figure 8.7 highlights the C and CO mole fraction at different pressures. A low-pressure operation is thermodynamically beneficial for lowering reaction temperature with reduced coke formation. Dry methane reforming at reduced pressure is worth investigating as long as mass processing and thus low cost are guaranteed.

8.3.2

Reaction Kinetics Over Supported Catalyst

Dry methane reforming over supported metal catalyst on various support materials has been investigated extensively and excellent review articles are available [63–65]. The reaction mechanism is explained by the Langmuir-Hinshelwood (L-H) mechanism as schematically depicted in Fig. 8.8. Briefly, CH4 is chemisorbed on metallic sites and almost irreversibly dehydrogenated towards the carbon atom (8.R18) [60]. Adsorbed carbon forms a solid solute with Ni and

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Fig. 8.7 Effect of pressure on C (solid carbon) and CO mole fraction

Fig. 8.8 A schematic representation of the Langmuir-Hinshelwood mechanism for dry methane reforming

may grow carbon filaments when the carbon concentration is saturated in Ni/NiC nanoparticles: CH4 ! CH4  ! CHx  ! C $ C  Ni ! carbon filament

ð8:R18Þ

Independently, CO2 is more likely to be adsorbed near the interface between the Ni catalyst and a metal oxide support such as Al2O3 [76]. Here, adsorbed species are denoted by  in figures and equations. The key step is that CHx is oxidized by CO2derived oxygen species to form CHxO before complete dehydrogenation to C occurs. Successful production of CHxO leads to syngas (CO and H2) without coke formation [77]. However, successive dehydrogenation of CHx to C is so fast that adsorbed carbon C becomes the abundant surface species [60]. Adsorbed carbon (C) readily combines with the transition metal catalyst and forms a core–shell like carbon-rich solid solute or molten thin layer on the surface of the catalyst nanoparticles [78]. Carbon diffusivity in the disordered molten layer is much faster than in the crystalline solid phase of the core region. Detailed kinetic

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analysis has been performed by Puretzky et al. within the scope of a carbon nanotube/nanofiber growth mechanism [79]. In the case of gas reforming, the surface carbon must be removed before it is saturated in the molten layer and grows carbon filaments. This phenomenon is known as coking, and it is purely detrimental because solid carbon blocks active sites and eventually clogs pellet pores. Although coke formation is observed frequently in CH4 reforming, this process is not included in the scope of the L-H mechanism: a theoretical formulation of DMR and SMR in the L-H scheme is employed without coke formation. Carbon oxidation is possible by a reverse Boudouard reaction (8.R19). However, this reaction is slow when compared to 8.R18: the overall carbon removal reaction is even more decelerated when the carbon diffusion within the catalyst particles becomes a rate-determining step [3]. The addition of H2O vapor is known to be effective in avoiding coke formation, which is explained by thermodynamic considerations [78]. In a nonthermal plasma hybrid reaction, plasmaactivated H2O promotes carbon oxidation (8.R20) and overall syngas productivity is increased with reduced coke as discussed in Sect. 8.2.2.1. A similar principle is applicable to DMR [74]: C þ CO2 ¼ CO þ CO C þ H2 O ¼ CO þ H2



ΔH ¼ 172 kJ=mol 

ΔH ¼ 131 kJ=mol

8.3.3

Surface Reaction Model

8.3.3.1

Langmuir-Hinshelwood Mechanism



ð8:R19Þ



ð8:R20Þ

ΔG ¼ 120 kJ=mol ΔG ¼ 91 kJ=mol

The overall reaction rate (r) for DMR is expressed by either Eq. 8.3 or Eq. 8.4 by taking into account 8.R16: r ¼ kPA n PB m r¼

1 d½CH4  1 d½CO2  1 d½ H 2  1 d½CO ¼ ¼ ¼ ν1 dt ν2 dt ν3 dt ν4 dt

ð8:3Þ ð8:4Þ

Here, P expresses a partial pressure of a gaseous species; subscript A and subscript B represent CH4 and CO2, respectively; k is the overall reaction rate constant; n and m, respectively, express reaction order for CH4 and CO2. In many cases, reaction order takes the value of n ¼ 1 and m ¼ 0 as shown in Fig. 8.9 [65]. Wang et al. examined CO production rate as a representative overall reaction rate of DMR (r) and found it is proportional to the CH4 partial pressure if coke formation is absent. In contrast, CO production rate is independent of CO2 partial pressure when it is sufficiently high. This expression is similar to SMR [60, 61].

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Fig. 8.9 Effect of CH4 and CO2 partial pressure on CO production rate, representing overall reaction order for DMR (Eq. 8.3) [65]

A heterogeneous L-H reaction is initiated with the adsorption of molecules onto the active sites of catalyst, followed by surface reactions and desorption to yield products. If CH4 and CO2 are adsorbed on the same active sites, the overall reaction rate is expressed by the following equation: r ¼ k s nA nB θ A θ B ¼ k s nA nB

K A K B PA PB ð1 þ K A PA þ K B PB Þ2

ð8:5Þ

Here, ks expresses the overall surface reaction rate constant; θ is surface coverage which takes the value between 0 and 1; K is the equilibrium constant between adsorption and desorption; n is the total number of active sites per unit area. K and θ are correlated with the Langmuir isotherm where adsorption and desorption are equilibrated. For catalysts commonly used in DMR, CH4 is selectively adsorbed on metallic sites and CO2 absorbs somewhat selectively on the interface between the metallic catalyst and the metal oxide support. The mathematical expression becomes even more complex if the WGS equilibrium, reverse DMR, and type of catalyst are considered [76]. For simplicity, WGS and reverse DMR are not considered. Further assuming that CO2 adsorbs only on metal-support interface, Eq. 8.5 is rearranged to Eq. 8.6: r ¼ k s nA nB

K A K B PA PB ð1 þ K A PA Þð1 þ K B PB Þ

ð8:6Þ

Assume CO2 absorption is strong and equilibrated, i.e., KBPB 1; Eq. 8.6 is approximated by Eq. 8.7: r ¼ k s nA nB

K A PA / PB 0 ð1 þ K A PA Þ

ð8:7Þ

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Equation 8.7 indicates that the reaction order is zeroth-order for PB (m ¼ 0 in Eq. 8.3); in other words, the overall DMR reaction rate is independent of the CO2 partial pressure. You can further assume KAPA 1, implying CH4 is weakly bounded by the metallic sites. Eventually, Eq. 8.7 simplifies to Eq. 8.8: r ¼ ks nA nB K A PA / PA

ð8:8Þ

The overall reaction rate is first-order in terms of CH4 partial pressure. The overall reaction rate (r) is also proportional to nA and nB: catalysts having large surface area for both CH4 and CO2 adsorption are highly desired for the fast reaction. There are two essential hypotheses: KAPA 1 and KBPB 1. A phenomenological understanding of the first hypothesis is that CH4 is weakly absorbed on metallic sites and CH4 dissociative absorption becomes the rate-determining step. The second hypothesis indicates that CO2 absorption is relatively fast. Active sites are readily occupied by CO2; therefore, CO2 partial pressure is independent of the overall reaction rate (r). The overall surface reaction rate constant (ks) is influenced by various elementary steps, and it is hard to identify individual contributions. It must be mentioned that Eq. 8.8 does not take coke formation kinetics into account. Meanwhile, plasma-activated H2O and CO2 play a critical role in oxidizing CH4 fragments and suppressing coke formation. For a better understanding of plasmaenhanced catalysis of DMR, the kinetic analysis of the rate-determining step and for the coke formation behavior is discussed in detail in Sect. 8.5.1.

8.3.3.2

Precursor-Mediated Mechanism

Plasma-activated species are involved in the heterogeneous surface reactions. If molecules are excited primarily before impinging on the catalysts surface and being trapped at an active site, adsorption cannot be explained on the basis of thermal equilibrium. In other words, surface coverage (θ) is unable to be formulated using the equilibrium constant (K ) within the scope of the Langmuir isotherm. This kind of adsorption process is categorized as a precursor-mediated mechanism as schematically presented in Fig. 8.10a. In the case of CH4 activation, the chemisorption probability of vibrationally excited CH4 is promoted exponentially with the internal energy [80]. If not the weakly bounded physiosorbed “hot CH4” (precursor state admolecules) would lose internal energy and desorb. This process becomes an extremely important elementary step in plasma catalysis because vibrationally excited molecules are produced by low-energy electron impacts and easily populated in plasma [54, 81]. However, it must be pointed out that the vibrational excitation of CH4 is overfocused in nonthermal plasma chemistry. As discussed previously, H2O activation also plays an essential role because plasma-activated H2O promotes oxidation of adsorbed carbon and regenerates active sites for the successive CH4 adsorption. Otherwise, active sites are fully occupied by the adsorbed carbon and it is hard for further CH4 conversion to take place. Likewise, we found experimentally that activated CO2 has an ability to react with surface

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Fig. 8.10 A schematic representation of (a) precursor-mediated adsorption and (b) Eley–Rideal mechanism

carbon species. As discussed in Sect. 8.3.3.1, it is evident that both CH4 and CO2 must be equally excited by nonthermal plasma so that the overall reaction rate is enhanced by nonthermal plasma. Another important issue is the possibility of an Eley–Rideal mechanism. Active sites are not always vacant but mostly occupied by some kind of species. In that case, adsorbed species may have a direct interaction with incoming plasma-activated molecules which carries the internal energy beyond thermal equilibrium. Such a reaction should be explained by an Eley–Rideal mechanism (Fig. 8.10b), rather than precursor-type adsorption enhancement. The role of vibrationally excited species within the scope of Eley–Rideal mechanism has yet to be discussed in detail in plasma catalysis.

8.4 8.4.1

Plasma Hybrid Reaction Packed-Bed DBD Reactor

A catalyst packed-bed DBD reactor is the most frequently used for hybrid operations. Figure 8.11 shows the hybrid reactor used for DMR in our study [75]. Briefly, the reactor consists of a 20 mm i.d. and 1.5 mm thick quartz tube, a 3 mm diameter centered electrode, and the external grounded electrode. The inner electrode was connected to a high-voltage power supply (Fig. 8.12). Although the voltage waveform is distorted from an ideal sine wave depending on the load impedance, it features compactness, low-cost, and a high-power output capacity (850 VA, Vpp ¼ 6~ + 8 kV, 13–14 kHz). The current waveform (Fig. 8.13a) exhibits spikelike current peaks at the rising and falling parts of the applied voltage, which is a standard characteristic of streamer-type DBD. Discharge power was measured by a voltage–charge (Lissajous) diagram [82] as shown in Fig. 8.13b. Spherical catalyst pellets (3 mm mean diameter, 12 wt.% Ni/Al2O3, Süd-Chemie) were packed for 40 mm length, and both ends of the catalyst bed were supported by metallic disks. The catalysts were reduced in a H2/Ar flow (100/900 cm3/min) at 600  C for 60 min before the reforming reaction. CH4, CO2, H2, and CO were quantitatively analyzed by a quadrupole mass spectrometer (QMS, Prisma; Pfeiffer Vacuum GmbH) after

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Fig. 8.11 Experimental setup: (a) overview of the packed-bed DBD reactor; (b, c) cross-sectional view at position (b) and (c)

H2O removal by a cold trap (ca. 233 K). Figure 8.13c–e represents an overview of the catalyst bed, optical emission, and the temperature distribution: the catalyst temperature clearly decreased due to the endothermic reforming reaction. The gas temperature was estimated by optical emission spectroscopy of the CO(B-A) transition [83]. The rotational temperature of CO and the catalyst temperature measured by thermography were well matched as discussed in Sect. 8.4.1.2. The reactor pressure was evacuated until it reached 5 kPa so that DBD is generated stably even if carbon is deposited on the catalyst pellets. Streamer-type gas breakdown initiates at the pellet contacts; subsequently, a number of filamentary discharge channels propagate covering a broader area of the pellet surface when the electrical conductivity of catalysts is low [84]. Coke formation increases electrical conductivity of pellet surface and streamer propagation is suppressed substantially.

8.4.1.1

Basic Property of DBD

Energy consumption of plasma gas conversion is benchmarked by the specific energy input (SEI):

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Fig. 8.12 The high-voltage power source used for DBD generation (Logy Electric, LHV-13 AC)

SEI ¼ C 

P ðeV=moleculeÞ F total

ð8:9Þ

C represents a conversion factor of the unit (cf. Appendix in Ref. [3]). The SEI is calculated unambiguously by the discharge power (P) and the total flow rate of feed gas (Ftotal) no matter what the reaction conditions would be. Therefore, performance levels of different conditions as well as different plasma reactors are classifiable on a SEI basis. According to the definition, the SEI expresses mean discharge energy consumption per unit volume of feed gas (e.g., J/cm3), which is also interpreted as mean energy fed into a single molecule (eV/molecule). The SEI is used as an indication of the energy efficiency of plasma processes. For example, the specific energy requirement (SER) for an endothermic reaction is readily obtained from the reaction enthalpy: for dry methane reforming, SER ¼ 247 kJ/mol ¼ 2.56 eV/molecule (8.R2). Assuming the plasma catalysis of DMR is operated at SEI ¼ 1 eV/ molecule, about 40% of energy required for DRM can be supplied from plasma and 60% is from ambient thermal energy. This estimation is apparently different from energy efficiency (cf. Sect. 8.5.2); the point is that the SEI is preferably as small as possible to minimize the energy penalty of plasma process. When the SEI is greater than the SER, the energy and material conversion efficiency is inevitably low and such conditions must be avoided. For an exothermic system, the definition of energy efficiency and relationship with SEI is discussed in ref. [50].

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Fig. 8.13 (a) Voltage and current waveforms; (b) a voltage–charge (Lissajous) diagram; discharge period “B-C” and “D-A”; non-discharge period “A-B” and “C-D”; (c) packed-bed DBD reactor; (d) DBD in the catalyst bed; (e) temperature distribution of the catalyst bed. Conditions: total flow rate ¼ 2000 cm 3/min (CH4/CO2 ¼ 0.5); P ¼ 89 W; Frequency ¼ 12 kHz; Pressure ¼ 5 kPa

The gas hourly space velocity (GHSV) is used commonly to express contact time, showing how many times of volumetric gaseous materials are processed with respect to the total volume of packed materials per unit time (h1) at standard condition (25  C and 101.3 kPa). Alternatively, weight hourly space velocity (WHSV) is used which is calculated from the mass flow rate of gaseous materials (g/h) divided by the total mass (weight) of catalyst materials packed into the reactor (g). GHSV and WHSV have the same unit (h1), which is apparently equivalent to the inverse of reaction time; therefore, for simplicity, we defined GHSV using the reactor volume (V ) as an alternative parameter of reaction time: GHSV ¼ ðReaction timeÞ1 ¼

F total
1  h V

The product of the GHSV and SEI is equivalent to the power density:

ð8:10Þ

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GHSV  SEI /

 P
W=cm3 V

ð8:11Þ

Power density is obtained by the discharge power and the reactor volume independently of the total gas flow rate. Assume the total gas flow rate is doubled; the energy density and reaction time (i.e., GHSV1) are halved simultaneously. Therefore, the conversion of initial feed may decrease by less than half. Such reaction behavior cannot be discussed by a power density basis. The individual contribution of SEI and GHSV must be taken into account for the appropriate analysis of reaction behavior. Power density (W/cm3) becomes the important parameter when designing a DBD reactor. DBD is characterized as a number of filamentary discharges with nanosecond durations. The charge accumulated on the dielectric barrier creates the reverse electric field, which suppresses the further development of weakly ionized streamertype discharges into an intense arc. Therefore, the energy fed into a DBD reactor, or power density, inevitably has an upper limit in terms of the capacitance of dielectric materials which is determined by the electrode configuration. Discharge power is given by Manley’s equation for the parallel-plate electrode configuration without packing materials between the gap [82]: 

P ¼ 4fV C d





  Cg  Vp  1 þ V ðW Þ Cd

ð8:12Þ

The f represents frequency; Cg and Cd, respectively, denote the capacitance of the gas gap and the dielectric material; Vp is the applied peak voltage and V expresses the effective gas breakdown voltage (cf. Fig. 8.13b). Assume Cd Cg, Eq. 8.12 reduces to Eq. 8.13 showing power density:     
 P V Cd
 4f V p  V  W=cm3 Ad d A

ð8:13Þ

Here, V/d represents the mean electric field strength between the gas gap; Cd/A is the capacitance of the dielectric material per unit area. Power density is determined by the number of streamers generated per unit area and unit time; a typical number density of streamer in air DBD is 106 cm2 s1 [85]. Equation 8.13 does not display parameters indicating the electron density and electron energy per streamer, or the streamer number density. All these important properties of DBD are merged in V and not decoupled. In many cases, we do not tailor V; but we merely measure V which is determined as a consequence of complex discharge events. Because V is fairly independent of Vp and takes a fixed value under the given conditions, power density increases only slightly by a lone tuning knob, i.e., Vp. The total number of streamers per half cycle increases with Vp, but streamer number density, electron energy, and electron density are essentially unchanged. Meanwhile, frequency ( f ) has a much greater impact than Vp; power density increases linearly with operating frequency, enabling one to design a high-power density and compact DBD reactor.

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Such peculiar characteristics of DBD have been reported by Kogelschatz in relation to ozone synthesis [66]. Later, Nozaki et al. demonstrated similar characteristics in CH4-fed DBD by the comprehensive energy balance analysis [2, 34]. It must be mentioned the discharge property of DBD has a strong correlation with gas composition: the discharge property at the inlet (e.g., CH4/CO2) and the outlet (e.g., CH4/CO2/H2/CO/H2O) of the reactor must be different to some extent. Meanwhile, SEI, GHSV, and the power density (i.e., V) represent time- and spaceaveraged properties of DBD; overly detailed interpretation of plasma chemistry with macroscopic parameters might lead to inconsistent inferences.

8.4.1.2

Catalyst Temperature Measurement

Temperature measurements of the catalyst bed are critically important because heterogeneous reactions must be characterized by the surface temperature. However, the catalyst temperature has not been reported in many literatures; either electrode temperature or furnace temperature was monitored. We paid special attention to the catalyst temperature characterization: catalyst temperature was monitored by thermography and maintained constant by an electric furnace before the reaction so that the initial condition is known and fixed. Upon introducing the CH4 and CO2 mixture, the catalyst temperature decreased clearly because of the endothermic reaction, while it increased to some extent due to the heat generated by DBD. The catalyst temperature is modified in this way and the overall heat transfer from the furnace to the catalyst is biased from the initial conditions. Eventually, a steady-state catalyst temperature is established by negative feedback of the energy balance between (i) the endothermic reaction enthalpy per unit time, (ii) discharge power, and (iii) the overall heat transfer between the furnace and the catalyst bed; (i) and (iii) are temperature-dependent, while (ii) is a temperature-independent process. The gas temperature difference between the inlet and the outlet of the reactor was omitted in the analysis as it was such a minor effect. It is quite an engineering-related subject, but we have to carefully analyze the energy balance in relation to catalyst temperature in order for a truly beneficial comparison between plasma and thermal catalysis. As can be seen in Fig. 8.13e, the individual catalyst pellet is identified in the thermal image, showing the outside layer of catalyst bed as the main part of the infrared emission. The transmittance of the quartz tube is generally higher than 90% when the wavelength is shorter than 3 μm, while the given thermography detected the infrared signal between 3.0 and 5.3 μm. It must be mentioned a thermography which detects long-wavelength (814 μm) measures the outside surface temperature of the quartz tube and individual catalyst pellets were unidentified by the thermal image. The thermography was calibrated by thermocouples without DBD under the reforming reaction. After steady state was established and making sure of a uniform temperature distribution by the thermal image, the emissivity of 0.82 provided the best correspondence between the thermocouples and thermography measurement. We confirmed that the emissivity (0.82) was unchanged for the reduced (Ni/Al2O3),

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Fig. 8.14 A comparison of catalyst temperatures measured by the thermography and the plasma gas temperature by CO emission spectrum fitting [83]

oxidized (NiO/Al2O3), and coked (C + Ni/Al2O3) catalysts. Infrared emission from DBD via the vibrational relaxation of molecules was not taken into account. The optical emission spectroscopy (OES) of the CO (B-A) transition was employed for gas temperature analysis during the reforming reaction [83]. Spectral fitting between 477 nm and 484 nm yielded the gas temperature close to the catalyst temperature with a smaller than 30 K difference (Fig. 8.14). Similar results were confirmed by the Boltzmann plot approach. DBD is characterized as a weakly ionized plasma with a nanosecond duration; temporal gas heating by the discharge propagation is several degrees higher than the ambient gas temperature [66], which is smaller than the measurement discrepancy between OES and thermography. In conclusion, temperature of catalyst pellets and plasma gas temperature are quasiequilibrated regardless of heat absorption by endothermic reaction and heat generation by DBD.

8.4.2

Backward Reactions

In this section, the backward reaction of DMR is discussed. Nonthermal plasma primarily promotes forward DMR (8.R2). However, as CO and H2 partial pressure increases under the presence of unreacted CO2 as well as H2O (cf. 8.R22.1), various side reactions occur simultaneously, and the individual reaction is influenced to some extent by nonthermal plasma. Important reactions are reverse DMR (8.R21) and the Sabatier reaction (8.R22):

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2CO þ 2H2 ¼ CH4 þ CO2

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ΔH ¼ 247 kJ=mol



ΔG ¼ 170 kJ=mol ð8:R21Þ

CO2 þ 4H2 ¼ CH4 þ 2H2 O





ΔH ¼ 165 kJ=mol

ΔG ¼ 113 kJ=mol ð8:R22Þ

The Sabatier reaction is a combination of the reverse water gas shift (RWGS) (8. R22.1) and methanation reactions (8.R22.2): CO2 þ H2 ¼ CO þ H2 O CO þ 3H2 ¼ CH4 þ H2 O



ΔH ¼ 41 kJ=mol 

ΔH ¼ 206 kJ=mol



ΔG ¼ 29 kJ=mol

ð8:R22:1Þ



ΔG ¼ 142 kJ=mol ð8:R22:2Þ

The methanation reaction is identical with reverse steam methane reforming (8.R1). As shown in 8.R21 and 8.R22.2, CH4 might be regenerated via the backward reactions of DMR and SMR, and the gross CH4 conversion may be suppressed at some point of reaction. The backward reactions are possible over a Ni/Al2O3 catalyst: indeed, supported nickel catalysts are the most commonly studied materials in the catalytic hydrogenation of CO2 [86]. Figure 8.15 compares the Gibbs free energy of the backward reactions. 8.R21 and 8.R22.2 are exothermic reactions and thermodynamically favored when the temperature is below ca. 600  C. In the case of low-temperature CH4 catalysis, CH4 regeneration might be promoted by DBD under the presence of Ni/Al2O3 catalysts. Therefore, 8.R21 and 8.R22.2 were examined using CO2 and H2 as initial feed at various temperatures. Figure 8.16 shows the reactant (CO2, H2) and product (CO, H2O, CH4) distribution obtained with and without DBD using a Ni/Al2O3 catalyst. The catalyst temperature was increased by a programmable electric furnace at a constant heating rate of 10  C/min, while the catalyst temperature was recorded by thermography at

Fig. 8.15 The Gibbs free energy of reverse DMR (8.R21), Sabatier reaction (8.R22), RWGS (8.R22.1), and methanation reaction (8.R22.2)

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Fig. 8.16 The relationship between the WGS equilibrium and backward reactions. GHSV ¼ 15,000 h1, H2/CO2 ¼ 2, total pressure ¼ 5 kPa. Dotted line, equilibrium; solid line, with DBD; hollow line, without DBD. H2O was estimated from the hydrogen balance (not a measured value)

1-min intervals. The stoichiometric H2/CO ratio for reverse DMR (8.R21) is H2/ CO ¼ 1, which is equivalent to the module (M ¼ 1) with initial H2/CO2 ¼ 2 (Eq. 8.14), because H2 is consumed by RWGS (8.R22.1) and the same amount of CO is added to the system (see Ref. [11] page 119): When H2 =CO2 ¼ 2,

M¼

½H2   ½CO2  2  1 ¼1 ¼ ½CO þ ½CO2  0 þ 1

ð8:14Þ

Likewise, the stoichiometric ratio of the methanation reaction (8.R22.2) is represented by H 2/CO2 ¼ 4, which leads to the equivalent H 2/CO ¼ 3, or M ¼ 3. Experiments with H 2/CO 2 ¼ 2 and 4 ideally cover the reactions from

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8.R21 to 8.R22.2. Steam (H 2O) was removed before QMS online analysis and not measured quantitatively, but H 2O selectivity was estimated from the hydrogen balance. The experimental results were compared with an equilibrium calculation. Solid carbon was not considered in the equilibrium calculation because the carbon balance among CH 4 , CO, and CO 2 from the experiment was more than 95% and no carbon deposition was detected. Reactions are generally slow at lower temperatures: there is a large offset between the equilibrium and experimental observation below 500  C. As catalyst temperature increases, H2 and CO2 start to be consumed and CO is produced from ca. 300  C via RWGS. Although a small amount of CH4 was produced by either 8.R21 or 8.R22.2, CH4 partial pressure is well below equilibrium when the catalyst temperature is lower than 500  C. At high temperature (>600  C), production of CH4 is not favored thermodynamically (cf. Fig. 8.15) and the RWGS governs the gas composition. It deduces important conclusions: first, reactions between CO2, CO, H2, H2O, and CH4 are equilibrated (>500  C) with the given H2/CO2 mixture as an initial feed. Moreover, there is little influence of DBD between 200 and 700  C for these reactions. Hereafter, this characteristic is called the WGS equilibrium. Second, CH4 regeneration is negligible between 200 and 700  C. CH4 conversion beyond equilibrium may be possible if the forward DMR reaction is promoted kinetically by nonthermal plasma. A similar trend was confirmed experimentally with an initial H2/ CO2 ¼ 4 (date is not shown).

8.4.3

Pulsed Reforming Spectrometry

We have developed pulsed reforming spectrometry which enables systematic analysis of DMR under the influence of nonthermal plasma. In order to overview the pulsed spectrometry, representative results are presented in Fig. 8.17 together with the optical emission spectroscopy data [3]. Pulsed CH4 injection was employed for 1 min at 3-min intervals, while CO2 was continuously supplied. Pulsed reforming was conducted over several cycles until the gas components and catalyst temperature reached a cyclic steady state. Before the CH4 injection (t < 0.2 min), Ni/Al2O3 catalysts would be almost fully adsorbed with CO2. The CH4 flow was turned on at t ¼ 0.2 min and the CH4 flowed over the CO2-adsorbed catalyst pellets: at this moment, H2 and CO are produced instantaneously, implying the CH4 dehydrogenation on metallic sites, formation of CHxO, and the desorption of H2 and CO are sufficiently fast reactions. Moreover, the reaction between H2 and CO2 should produce CO which is governed by the WGS equilibrium as discussed in detail in Sect. 8.4.2. H2 partial pressure monotonically decreased during the reforming period probably because the catalyst temperature decreases due to the endothermic nature of DMR. After 1 min into the reforming reaction, the CH4 flow is turned off at t ¼ 1.2 min and CO2 only DBD was generated for 3 mins (1.2 < t < 4.2 min). Although CH4 is absent, CO2 is consumed by the reverse Boudouard reaction (8.R19) and CO partial pressure

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Fig. 8.17 The emission intensity of CO (518.3 nm) (solid blue line) and C2 (dotted black line) at (a) the reactor inlet and (b) the outlet. (c) Mass spectrum profile for one cycle pulsed reforming. The emission profile is normalized by the emission intensity of C2 peak at 1.25 min [3]. CH4/CO2 ¼ 2, power 70 W

increases correspondingly. By integrating the CO2 signal over the de-coking period, solid carbon formation during the 1-min reforming period is calculated quantitatively. Unlike the reforming reaction, carbon oxidation by CO2 is a slow reaction; therefore, the CO and CO2 profiles change quite moderately and carbon removal requires ca. 1.4 min (1.2 < t < 2.6 min). The time-dependent change of the line intensity of CO (519.8 nm) and the C2 high-pressure Swan system (589 nm) are synchronized with the pulsed reforming. The emission profile is normalized by the intensity of the C2 peak appearing at t ¼ 1.25 min. At the reactor inlet (Fig. 8.17a), emission of CO is weak and almost absent in the de-coking period simply because CH4 and CO2 conversion is small. Emission from C2 is negligibly small over the whole cycle at the reactor inlet. The CO profile at the reactor outlet (Fig. 8.17b) is well correlated with the CO mass spectrum profile over the cycle. This is because measurement by mass spectrometer directly reflects gas composition at the reactor outlet and the intensity of CO emission simply proportional to the CO partial pressure. In contrast, the C2 emission profile is quite unique: C2 emission intensity is weak during the reforming period and increased momentarily at t ¼ 1.25 min, which is slightly after the CH4 flow was turned off.

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By comparing Fig. 8.17b, c, C2 emission is observed only when CO exists. The intense C2 peak at t ¼ 1.25 min indicates that coke gasification by the reverse Boudouard reaction increase the CO partial pressure abruptly. Note, a sharp increase in CO at t ¼ 1.25 min is not observed because CO is consumed to produce C2. The C2 emission is observed until t ¼ 2.6 min under the presence of CO. The reaction mechanism from CO to C2 is discussed in detail by S Kameshima et al. [3]. Although CO2 DBD is generated between 2.6 < t < 4.2 min, CO is not detected; thus the C2 emission is absent. The specific energy input in this experiment is ca. 1 eV/molecule, and this value is too small to dissociate more than 1% of the CO2 (CO2 + e ! CO + O + e) in the gas phase [87]. The experimental observations lead to important conclusions: first, the excitation of CO2, rather than dissociation, is the key reaction pathway for plasma hybrid heterogeneous conversion of CO2 and CH4. Second, heterogeneous oxidation of adsorbed carbon is well characterized by the gas-phase C2 emission, which is utilized for a better understanding of the plasmaenhanced surface reaction mechanism.

8.4.4

Other Reactions

In this experiment, higher hydrocarbons, such as C2H6 and C3H8, were not synthesized. The higher hydrocarbons were presumably produced by DBD in the gas phase via dimerization of CiHj radicals; however, those species were fully converted to H2 and CO at relatively high catalyst temperatures. The key influencing factor is the specific energy input. The production of higher hydrocarbons has been reported in packed-bed DBD reforming of CO2/CH4 [67–69] where the SEI was varied between 4 and 33 eV/molecule [67], 20–28 eV/molecule [68], and 1.4 eV/molecules [69]. An excessive energy input leads to the production of non-negligible amounts of higher hydrocarbons. Moreover, the complete conversion of higher hydrocarbons to syngas would not be possible at low catalyst temperatures (230  C [67], unknown [68], 50–230  C [69]). In contrast, the SEI in this experiment (1 eV/molecule) was sufficiently small and catalyst temperature (500–600  C) is higher than that reported in the literature. In summary, H2, CO, H2O, and C were the major products, and the net production of higher hydrocarbons was absent.

8.5 8.5.1

Reaction Mechanism Kinetic Analysis

In the thermal catalysis, the kinetic analysis is performed with either fixed CH4 and variable CO2 partial pressures or vice versa using N2 or Ar as an inert balance. However, neither N2 nor Ar dilution is suitable for plasma catalysis: excited N2 and Ar are even more reactive than excited CH4 and CO2. Gas phase and surface

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chemistry would be biased by the unrecognized contribution of those species. Therefore, we must establish a reaction diagnostic method without a dilution gas. For this reason, multiple-pulsed DMR was performed by systematic variation of the CH4/CO2 ratio without dilution gas. CH4 and CO2 flow rates were controlled individually by programmable digital mass flow controllers. The CH4/CO2 ratio was initially set to 0.5. Every three-cycle operation, the CH4/CO2 ratio was incremented stepwise until CH4/CO2 ¼ 2 consecutively, while the total flow rate was fixed at 2000 cm3/min (GHSV ¼ 10,300 h1). The discharge power was 85–90 W where specific energy input was ca. 0.6 eV/molecule [74]. Reactant conversion and product yields are shown in Fig. 8.18. The definition for conversion and yield is provided in S Kameshima’s research [74]. Figure 8.18a shows that the CH4 conversion in the hybrid reaction was the same as that in the thermal reaction, implying that CH4 dehydrogenation (8.R18) was not affected by DBD. Nevertheless, the H2 yield was clearly increased by the hybrid reaction. A possible explanation is that the plasma-activated H2O oxidizes adsorbed carbon (8. R20) to produce H2. The reactivity of plasma-activated H2O was studied by an Arrhenius plot analysis where reaction order for H2O was doubled by DBD [61]. As a result, only the H2 yield increased without increased CH4 and CO2 conversion when CH4/CO2 < 0.67. In contrast, the CO2 conversion and the CO and H2 yields were clearly promoted by DBD at CH4/CO2 > 0.67. Plasma-activated CO2 and H2O would promote surface reaction and increase CO and H2 yield. The experimental observation derives a tentative reaction scheme illustrated in Fig. 8.19. As discussed in Sect. 8.4.2, H2O, CO, H2, and CO2 establish a WGS equilibrium with and without DBD. However, if plasma-activated H2O promotes the surface reaction with adsorbed carbon (C), it creates additional pathways to syngas (8.R20). Similarly, the reaction between plasma-activated CO2 and C increases CO yield (8.R19). In fact, coke formation is suppressed by DBD (Fig. 8.18e): a similar result was reported in CH4 steam reforming [53] and further confirmed in CH4 dry reforming [75]. Coke (C) oxidation would proceed irreversibly and CO partial pressure increases rather selectively. Subsequently, due to the WGS equilibrium, product distribution is rearranged toward “CO2 and H2,” producing more H2 than CO when the temperature is well below 800  C (cf. Fig. 8.15 and 8. R22.1). It was proven by the experiment that H2 yield was enhanced much more than CO yield at 550  C (Fig. 8.18c, d). Although production of C is detrimental for catalyst activity and lifetime, presence of C creates key pathways for emerging plasma-induced synergistic effect. As for CH4 conversion, a synergistic effect was not observed clearly. Although experiments were carried out at fixed catalyst temperatures of ca. 550  C, there was a possibility that the synergistic effect may occur at a different temperature range.

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Fig. 8.18 The effect of the CH4/CO2 ratio on DMR at 550  C: (a) CH4 conversion; (b) CO2 conversion; (c) H2 yield; (d) CO yield; (e) coking rate. Open circle, thermal reaction (w/o DBD); solid circle, hybrid reaction

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Fig. 8.19 A tentative reaction scheme of the DBD induced synergistic effect. Gasification of solid carbon by plasma-activated H2O and CO2 biased WGS equilibrium toward “CO2 + H2,” yielding more H2 than CO

Fig. 8.20 (a) Mean energy absorption by DMR and (b) energy efficiency. Open circle, thermal reaction (w/o DBD); solid circle, hybrid reaction

8.5.2

Energy Efficiency

The average energy absorption by DMR (ΔE) over the reforming period (0 < t < τ) and the energy efficiency (η) were calculated by the following equations: ΔE η¼ ðÞ P Z 1 τ ΔE ¼ ΔEðt Þdt ðW Þ τ 0 





ð8:15Þ ð8:16Þ 

ΔE ðt Þ ¼ ν6 ΔH H2 Ojvapor þ ν4 ΔH CO  ν1 ΔH CH4  ν2 ΔH CO2 

ð8:17Þ

Here, P (W) is discharge power and ΔH (J/mol) represents standard enthalpy of   formation (ΔH C ¼ ΔH H2 ¼ 0 J/mol). The stoichiometric coefficient appearing in Eq. 8.17 (νi, mol/s) was obtained experimentally. Figure 8.20a shows that ΔE

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increases by DBD, especially in region B where plasma-induced synergy clearly appears. According to the definition, enhancement of ΔE is due to increased CO2 conversion (the fourth term in Eq. 8.17) and H2 selectivity (i.e., decreased H2O selectivity, the first term). CH4 conversion was not increased by DBD and net zero contribution to ΔE. The increased CO selectivity (the second term) is a negative  contribution to the overall energy absorption (ΔE) because ΔH CO < 0. As discussed in Fig. 8.19, the increase in H2 selectivity in the low-temperature plasma catalysis is much greater than that of CO, which is beneficial to increase energy efficiency. Energy efficiency reached 60–65% in region A; however, it decreases monotonically in region B, simply because CO2 conversion decreases with CH4/CO2 ratio and the  fourth term in Eq. 8.17 (ν2 H CO2 > 0) has a great impact on both ΔE and η.

8.6

Conclusion and Future Outlook

Nonthermal plasma activation of CH4 was overviewed, followed by a rather dedicated description of CH4 and CO2 reforming by DBD-enhanced heterogeneous catalysis. The nonthermal plasma hybrid reaction is explained based on the Langmuir-Hinshelwood (L-H) mechanism. The L-H mechanism consists of three steps: (I) adsorption of reactants, (II) surface diffusion/reaction, and (III) desorption of products. The rate-limiting step in CH4 steam/dry reforming is the dissociation of the strong C-H bond over the metallic sites of the catalyst. By the simple kinetic analysis, vibrationally excited CH4 is the most abundant and long-lived species produced by low-energy electron impact. Moreover, vibrationally excited CH4 is known to promote dissociative chemisorption over the metal surface. In this sense, the role of nonthermal plasma in the initial stage of the L-H mechanism is quite clear. However, there is an inconsistency between macroscopic kinetic analysis and theoretical prediction. Based on our extensive study, activation of H2O and CO2 creates the key reaction pathways for the emerging synergistic effect. Meanwhile, reaction enhancement via CH4 activation was not clearly identified in either steam or dry methane reforming. In addition to CH4 activation, we need to study the vibrationally excited CO2 and H2O, as well as their interactions with the catalytic surface. The role of nonthermal plasma on the second and third stage of the L-H kinetics is not well understood because nonthermal plasma has a limited interaction with the catalyst surface. For example, a plasma sheath is not generated clearly in the highly transient streamer-type discharge. Second, energy transfer from energetic ions to the surface is not expected in highpressure conditions. Finally, nonthermal plasma is not generated in microand nanometer pores; excited species might be able to penetrate into catalyst pores of the order of 1–100 micrometers [88, 89]. We hypothesize that the plasma-induced nonthermal heating mechanism would play a key role for the promotion of overall surface reaction including diffusion of adsorbates and

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desorption of products. Charge recombination and association of radicals on the catalyst surface can release energy corresponding to 1–10 eV/molecule. If this energy is transferred directly to the admolecules, surface reaction/diffusion may be enhanced without increasing the macroscopic catalyst temperature. This reaction scheme would be explained by the Eley-Rideal mechanism rather than precursor-type adsorption enhancement (cf. Fig. 8.10). Alternatively, such an energy release may increase the local temperature on the micro-, nano-, and even molecular-scale and promotes surface chemistry. Although the plasmainduced energy transfer mechanism is commonly accepted in particle growth on the micrometer [90] and nanometer scale [91], it has yet to be investigated within the scope of plasma catalysis. Kortshagen et al. estimated that the temperature of the free-standing silicon nanoparticles is likely to be equilibrated with the ambient gas temperature when particles grow beyond 10 nm [91], while the temperature of smaller particles (smaller than 5 nm) is elevated beyond the gas temperature. The heating and cooling events occur on the nanoparticle surface, while net temperature increase is determined by the gross energy gain divided by the volumetric heat capacity of particles. Inevitably, the surface to volume ratio of nanoparticles is the critically important parameter. In the case of supported catalysts, however, heat or energy readily escapes toward supporting materials. Therefore, the temperature of nanoparticle catalysts may be determined unambiguously by the temperature of supporting material no matter what their sizes are. A deep understanding of highly transient and nonequilibrium energy transfer via excited molecules, without macroscopic temperature change, needs to be achieved. Exploring new types of catalysts, dedicated to plasma catalysis, is an important subject of research. This is one missing research thrust in the plasma catalysis community. In looking toward the successful catalyst design and selection of appropriate materials, materials science research should be supported by the deep understanding of plasma chemistry, diagnostics, and modeling of interfacial phenomena [92]. Performance of newly developed catalysts is benchmarked by the macroscopic diagnostics method [2, 3, 75]. These approaches should be integrated as a plasma catalysis science platform, leading to truly beneficial applications. Acknowledgments This work is supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (JP16J09876). S.K. acknowledges JSPS for providing Research Fellowship for Young Scientists (DC1); Z.R. acknowledges financial support from the program of China Scholarships Council (No.201707040056). T.N. would like to thank Mr. Tinnapop Moonmuang (Mechanical Engineering, Chiang Mai University) for the experimental support.

References 1. Schiebahn, S., Grube, T., Robinius, M., Zhao, L., Otto, A., Kumar, B., Weber, M., & Stolten, D. (2013). In D. Stolten & V. Scherer (Eds.), Transition to renewable energy systems (pp. 813–847). Wiley-VCH.

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Chapter 9

Plasma-Catalytic Conversion of Carbon Dioxide Bryony Ashford, Yaolin Wang, Li Wang, and Xin Tu

9.1

Introduction

The emission of CO2 is a pressing concern as its release into the atmosphere is a major source of global warming. As global temperatures rise due to the greenhouse effect and current technologies, such as carbon capture and storage (CCS) and a switch to renewables, fall short, expertise must be employed to find new, viable processes for the mitigation of CO2. Focus is now on carbon dioxide utilization, as high-value chemicals and fuels can be produced, creating viable and sustainable processes. Current processes, however, such as thermal catalytic and electrochemical processes, require elevated temperatures and are not thermodynamically efficient, thus reducing their energy efficiency and feasibility. Plasma-catalytic processes have the potential to overcome these drawbacks due to their low-temperature operation and non-equilibrium characteristics which allow the high stability of the CO2 molecule to be overcome without the need for large energy inputs. Alongside this, the catalyst acts to lower the activation barrier and enhance the selectivity to the required product. Interactions also occur between the catalyst and the plasma, creating synergy. Furthermore, quick start-up and shutdown enable plasma-catalytic processes to be used as a method of storing excess energy from renewable energy generation. A great number of reactions can potentially be carried B. Ashford · Y. Wang · X. Tu (*) Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, UK e-mail: [email protected] L. Wang State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning, China College of Environmental Sciences and Engineering, Dalian Maritime University, Dalian, Liaoning, China Department of Electrical Engineering and Electronics, University of Liverpool, Liverpool, UK © Springer Nature Switzerland AG 2019 X. Tu et al. (eds.), Plasma Catalysis, Springer Series on Atomic, Optical, and Plasma Physics 106, https://doi.org/10.1007/978-3-030-05189-1_9

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out in a plasma-catalytic reactor, including CO2 decomposition, dry reforming of methane and CO2 hydrogenation; hence a great number of high-value products can be created (oxygenates, liquid hydrocarbons, syngas, etc.). Currently, a trade-off exists between high energy efficiency and high reactant conversion. This is due to conversion increasing with input power, which leads to a decrease in energy efficiency. A greater understanding of the plasma chemistry and the interactions between plasma and catalyst will further increase the viability of plasma-catalytic processes for the utilization of CO2 on an industrial scale. The following chapter describes this process in detail for a number of different reactions and discusses recent advances in the area.

9.1.1

Carbon Dioxide Emission

Carbon dioxide is a major greenhouse gas and is largely responsible for the changes we are currently seeing in the climate. Since the industrial revolution, greenhouse gas emissions have risen, with a 75% global increase in greenhouse gases occurring since 1970. In 2010, 49
109 tonnes of CO2 equivalent (GtCO2eq) of greenhouse gases were emitted from anthropogenic sources, of which CO2 sources currently make up the majority at 76% [1]. Industrial development and the release of carbon dioxide have always gone hand in hand, and with many countries still developing their industry and a rising global population, carbon dioxide emissions will continue to increase. As is well documented, a rise in CO2 in the atmosphere leads to warming of the planet, causing a rise in sea level which can wipe out whole communities in low-lying areas and increasing the frequency of disasters such as tsunamis and forest fires. In recent years, these disasters have become more commonplace, resulting in societal values changing to reflect the growing urgency of the situation and forcing governments around the world to take action against climate change. This need for change culminated in the Paris Agreement, a legally binding document between 195 countries that aims to tackle climate change, aiming to limit the global temperature rise to well below 2  C above pre-industrial levels. In order to fulfill targets set out in the agreement, a switch to renewable energy and a reduction in the release of greenhouse gases into the atmosphere are required, necessitating the design of novel technologies that enable this change while allowing society to prosper. The utilization of carbon dioxide from waste gas streams is one area which has the potential to fulfill these requirements, as CO2 from fossil fuel and industrial processes accounts for 65% of total annual anthropogenic greenhouse gas emissions, as detailed in the Fifth Assessment Report of Intergovernmental Panel on Climate Change (IPCC) [1]. A significant reduction in carbon dioxide emissions can therefore be made by capturing CO2 from waste gas streams and converting it into valuable fuels and chemicals.

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9.1.2

273

Current Technologies to Reduce CO2 Emission

There are several methods that can be used to convert carbon dioxide, including catalytic conversion, photocatalytic/photochemical processes, electrocatalytic/electrochemical processes, enzymatic/biochemical processes and plasma processes. Each of these methods results in a slight variation in the product created, with hydrocarbons, hydrogen and oxygenates forming via catalytic conversion, while carbon monoxide, hydrocarbons, syngas and oxygenates are the main products of plasma processes. Along with the need for high temperatures in thermal-catalytic processes, issues can be encountered with catalyst deactivation due to coking. Electrocatalytic processes show promise for producing methanol, formic acid and a variety of other organic chemicals. However, these processes have low thermodynamic efficiency. The least researched, yet predicted to be the most effective, is the plasma process [2, 3]. In comparison to the other processes, it is simple and fast: plasma has the potential to enable thermodynamically unfavorable chemical reactions (e.g. CO2 dissociation) to occur at ambient conditions [3].

9.1.3

Carbon Dioxide Utilization Through Plasma Technology

Nonthermal plasma (NTP) can be operated at room temperature and atmospheric pressure while still generating highly active species and electrons, with mean electron energy between 1 and 10 eV. This electron energy is the optimum range for exciting molecular and atomic species and breaking chemical bonds. For CO2 dissociation (9.R1) to occur in plasma, only 5.5 eV is required to break the OC ¼ O bond via stepwise vibrational excitation. Nonthermal plasma therefore shows great potential in the production of an efficient CO2 utilization process, as it can overcome the stability of CO2 without the need for the high temperatures required in thermal catalytic processes. Plasma technology is also advantageous over thermal processes as reaction rates are high and steady state is quickly reached [4]. This facilitates quick start-up and shutdown, a promising feature that enables plasma technology powered by renewable energy to act as efficient chemical energy storage through a localized or distributed system at peak grid times [5]. Different routes for CO2 conversion have been explored using NTP (Fig. 9.1).

9.2

Plasma CO2 Decomposition 2CO2 ! 2CO þ O2

ð9:R1Þ

Decomposition of CO2 into CO and O2 using NTP has recently attracted significant interest as this reaction is almost impossible at low temperatures using

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Fig. 9.1 Different routes for CO2 conversion

conventional catalysis. Furthermore, the product CO is an important chemical feedstock for further synthesis of fuels and chemicals. Various plasma systems are reported to successfully convert CO2 into CO and O2 (9.R1), including glow discharge, where one study found a CO2 conversion of 30% is achievable at an input voltage of 7 kV [6]; radio frequency discharge, which can achieve 90% CO2 conversion at 1 kW [7]; and microwave discharge, in which a 100 W power input was accompanied by a 90% conversion [8]. A carrier gas, such as helium or argon, has previously been commonplace in the dissociation of CO2 via plasma systems; however, this leads to an additional, undesired cost. Dielectric barrier discharge (DBD) reactors have been shown to successfully dissociate CO2 in the absence of a carrier gas [9, 10], with one study achieving 30% conversion at a power density of 14.75 W/cm3 [10]. In plasma, reactions mainly occur in the gas phase. Firstly, CO2 is dissociated into CO and an oxygen atom. The oxygen atom created then either combines with another oxygen atom to form molecular oxygen (9.R2), or it reacts with CO2 to form carbon monoxide and an oxygen molecule (9.R3): O þ O ! O2

ð9:R2Þ

O þ CO2 ! O2 þ CO

ð9:R3Þ

The production of carbon can also occur (9.R4), along with reverse CO2 decomposition (9.R5): CO þ e ! C þ O þ e

ð9:R4Þ

CO þ O ! CO2

ð9:R5Þ

CO2 conversion mainly occurs via electronic dissociation, vibrational excitation and dissociative attachment [11]. A zero-dimensional chemical kinetics model has

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been developed to understand the reaction mechanisms in the plasma CO2 dissociation [12]. Electron impact dissociation has been found dominant in CO2 decomposition in DBD plasmas, leading to lower energy efficiency compared to microwave plasmas or gliding arc discharges, in which more energy-efficient vibrational excitation of CO2 plays a key role in the decomposition of CO2 [13–15]. A 1D model developed for an AC gliding arc reactor has shown that dissociation only occurs at the center of the arc where the gas, vibrational and electron temperatures, as well as the ionization degree, are at their maximums [13]. The dissociation of CO2 in a gliding arc therefore only occurs significantly at the arc center. In a typical microwave plasma, gas temperature has shown to be an important factor in determining CO2 conversion, with a higher temperature resulting in a greater conversion; as temperature increases, the reaction rate coefficients for heavy-particle dissociation also increase, along with the number of vibrationally excited species [16]. The reformation of CO2 can be an issue in plasma reactors, decreasing the effective CO2 conversion as CO recombines with oxygen species (O2, O and O2) [17]. A novel approach to solving this problem was carried out by Mori and Tun, in which a DBD reactor was combined with a solid oxide electrolyser cell (SOEC) [17]. The SOEC was used to remove oxygen from the gas, hence restricting it from reacting with CO. The SOEC also contributed to CO2 splitting [17]. When the two were used separately for this reaction, the SOEC process reached a maximum conversion of 3%, while the plasma-alone process had an optimum conversion of 15%. However, when combined, a synergistic effect occurred as the hybrid system reached a conversion of 93%, which was attributed to the SOEC removing oxygen from the system, as shown in Fig. 9.2 [17]. It is novel ideas such as this which can greatly increase the feasibility of plasma CO2 decomposition on an industrial scale.

Fig. 9.2 CO2 conversion and outlet O2 mole fraction with residence time (plasma input power, 30 W; SOEC applied voltage, 7 V; SOEC current, 37–50 mA; CO2 flow rate, 0.23–18 ml/min; pressure, 3 kPa; inside heater temperature, 800  C; outside furnace temperature, 200  C) [17]

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Influence of Process Parameters

A design of experiments approach using a cylindrical DBD revealed that the main parameter which affects the energy efficiency of this process is the feed flow rate, while discharge power has the greatest influence on CO2 conversion [18]. A high feed flow rate leads to a lower CO2 conversion but is more energy efficient [18]. A high feed flow rate corresponds to a lower residence time, resulting in fewer interactions between the feed gases and the excited species, hence a lower conversion; however, a higher flow rate leads to lower specific energy input (SEI) (at constant plasma power) (Eqs. 9.1 and 9.2) and thus results in a more energyefficient process (Eq. 9.3) (equations found in [19]) [12]:


 Power ½kW SEI ½kJ=L ¼
60 ½s= min  Flow ½L= min  Z T ðV ðt Þ
I ðt ÞÞdt Power ¼ ð1=T Þ

ð9:1Þ ð9:2Þ

0

E½% ¼

ΔH r ½kJ=mol
X CO2 ½% SEI½kJ=L
molar volume½L=mol

ð9:3Þ

A trade-off therefore exists between energy efficiency and conversion, as energy efficiency decreases with increasing SEI, but conversion rises [12]. However, although SEI can remain constant when varying both plasma power and residence time simultaneously, a constant SEI does not necessarily result in the same values of CO2 conversion and energy efficiency. In fact, it has been found that obtaining the same value of SEI using varying combinations of residence time and plasma power can lead to changes in reaction performance [12]. At high residence time and low plasma power, the maximum CO2 conversion reached can be greater than when a low residence time and high power, but the same SEI, are used [12]. The effect of residence time on the conversion and energy efficiency is therefore greater than the effect of the plasma power. This is due to the length of time CO2 stays within streamers (longer for a high residence time) being the major influencing factor on conversion, as opposed to high streamer intensity (as a result of high plasma power). At high SEIs, energy efficiency and conversion can therefore be increased simultaneously; however, energy efficiencies will be low even when maximized [12]. Discharge gap can play an important role in determining CO2 conversion and energy efficiency. At constant SEI, an increase in discharge gap can lead to a decrease in energy efficiency and conversion if the increase is large enough to alter the streamer behavior [12, 20]. Smaller discharge gaps allow more streamers with higher peak currents to form; therefore, an increase in CO2 dissociation occurs due to the greater effective plasma volume, while the increase in electron density resulting from the higher peak currents leads to a rise in electron impact reaction rates [12]. Alongside this, the average electron energy will be larger for smaller gaps; hence when collisions occur, more energy will be transferred [12].

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The influence of discharge length has also been evaluated, with results showing that an increase in both discharge length and discharge power leads to an increased CO2 conversion [20, 21]. This is due to higher discharge power resulting in an increase in the number of micro-discharges, thereby increasing the number of reaction channels for collisions to occur [20, 22]. However, this only appears to be the case up to a certain point, with one study finding an increase in applied power above 80 W results in a decreased conversion, possibly due to a change in discharge mode from surface discharge to filamentary discharge [19]. Dielectric material and frequency have been found to have no influence on the energy efficiency and CO2 conversion [12, 20]. Alumina and quartz dielectrics were compared at various SEI values with results showing no significant differences in reaction performance between the two [12]. By contrast, Mei and Tu al reported that the thickness of dielectric materials affects the plasma conversion of CO2 and energy efficiency using a DBD reactor [20]. Increasing the thickness of a quartz tube from 1.5 to 2.5 mm decreased the CO2 conversion and energy efficiency of the plasma process by around 15% at a SEI of 120 kJ/L and a constant discharge gap of 2.5 mm [20]. In addition, they found that using a screw-type inner electrode in the DBD reactor significantly enhanced the conversion of CO2 and energy efficiency compared to the reaction using a rod electrode [20]. The sharp edge of the screw-type electrode could distort the electric field and enhance the local electric field around the inner electrode and consequently generate more intensified filamentary discharge which can also be evidenced by increased amplitude and number of current pulses. This effect could generate more reaction channels for CO2 conversion and makes a contribution to the enhanced reaction performance [20] (Fig. 9.3).

Fig. 9.3 Images of the CO2 DBD plasma: (a) rod inner electrode; (b) screw-type inner electrode (discharge power, 40 W; discharge gap, 2.5 mm; discharge length, 100 mm; CO2 feed flow rate, 25 mL/min; frequency, 9 kHz; outer electrode, stainless steel mesh) [20]

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A diluent gas, such as helium or argon, can be used in the feed alongside CO2. The addition of such gases can lead to a higher absolute CO2 conversion in a DBD; however, if total feed rate is kept constant, the actual amount of CO2 converted (effective CO2 conversion) will be lower in the mixed feed (CO2 + diluent) than for a pure CO2 feed due to the decrease in CO2 present in the feed [23]. The increase in absolute CO2 conversion in the diluted feed can be explained by the lower breakdown voltage and the increased plasma formation due to an increase in plasma capacity; the addition of helium causes a more homogeneous plasma to form, while argon results in a greater micro discharge filament density [23]. As the threshold energies for inelastic collisions involving He and Ar are much higher than those for CO2, and electron recombination is favored for CO2+ ions over Ar+ and He+, a greater fraction of plasma power will go into dissociating CO2 and less will go into gas breakdown as this can occur at a lower voltage due to an increase in the electron mean free path [23]. As well as increasing conversion, this also leads to an increase in energy efficiency. However, as a large fraction of the input energy goes into exciting the diluent gas, the effective energy efficiency drops in comparison to the pure CO2 feed [23]. A different effect results when N2, an impurity present in many waste gas streams, is added to the pure CO2 feed in DBD plasma. Below 50% N2, the absolute CO2 conversion increases as N2 molecules enhance conversion due to the collision between N2 metastable molecules and CO2 resulting in CO2 dissociation; thus, the effective conversion is tantamount to that of pure CO2 as the increase in absolute CO2 conversion cancels out the decrease in CO2 in the feed [24]. However, above 50% N2, a greater fraction of the input energy is transferred to N2 molecules rather than going into CO2 dissociation; hence effective CO2 conversion and energy efficiency decrease [24]. The effect of N2 addition to the feed differs in microwave plasma. Here, the effect on CO2 conversion and energy efficiency is similar to that found when Ar or He is added to the feed in DBD plasma: absolute CO2 conversion increases in comparison to pure CO2 feed; however the reduction in CO2 concentration results in a lower effective conversion and energy efficiency [25]. Absolute CO2 conversion increases with a rise in N2 concentration due to partial conversion of lower CO2 vibrational levels into higher ones [25]. An important point to note is that on addition of N2 at all concentrations to both microwave and DBD systems, harmful gases such as N2O and NOx are formed [24, 25].

9.2.2

Influence of Packing and Catalytic Materials

In current plasma systems, a trade-off exists between CO2 conversion and energy efficiency [7, 26]. In order to solve this problem and hence create a feasible industrial process, further modification of the plasma system is required. One such modification is the addition of a catalyst into the plasma discharge, as research shows the hybrid plasma-catalytic process can result in higher CO2 conversion while maintaining low energy consumption [19, 27]. The combination of plasma and

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Fig. 9.4 Single-stage setup in a plasma-catalytic DBD reactor

Fig. 9.5 Two stage setup in a plasma-catalytic DBD reactor

catalyst allows the beneficial aspects of each to be realized, along with the effect resulting from their interaction [28, 29]. This can lead to synergy in relation to conversion and efficiency, thus creating a more feasible process for the utilization of CO2 on an industrial scale. When a catalyst is combined with plasma, the reactions that occur change from purely gas phase to a mixture of gaseous and heterogeneous [11]. The simplest method of combining plasma and catalyst in a single-stage setup is to do so in a DBD reactor (Fig. 9.4), as the catalyst can be placed directly into the discharge without the need for any adjustments in reactor geometry. In this setup, the catalyst is in direct contact with the plasma and can therefore interact with shortlived active species such as excited-state atoms and molecules, reactive radicals, photons and electrons. In the two-stage setup (Fig. 9.5), a catalyst is placed downstream of the plasma discharge. The catalyst is not in direct contact with the plasma; hence it cannot interact with short-lived excited species, only with the exit gas which contains longlived intermediates and, possibly, vibrationally excited species. Catalysts can be incorporated into DBD reactors in a fully or partially packed-bed configuration [30, 31]. By mixing the catalyst and cheap packing material (e.g. glass beads, Al2O3 and quartz wool) before placing in the reactor, a packed-bed effect can also be realized without the need for high volumes of costly catalyst in addition to a catalytic effect. This setup also results in a quasi-homogeneous dispersion of the catalyst, which benefits the reaction performance as a greater number of CO2 molecules will come into contact with the catalyst. The packing material can also lead to an increase in both CO2 conversion and energy efficiency simultaneously. Zirconia beads with diameters in the range of 1.6–1.8 mm have been shown to increase CO2 conversion by almost 100% while nearly doubling the energy efficiency in comparison to values obtained in the absence of any packing material [19]. The presence of a packing material will decrease the residence time of CO2 molecules in the plasma when a fully packed bed is used. As mentioned in Sect.

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9.2.2 on the effect of various process parameters, a decreased residence time results in a decrease in CO2 conversion as there is less time for the feed gases and excited species to interact and hence for CO2 splitting to occur. It would therefore be assumed that packing material would cause a reduction in CO2 conversion. However, as CO2 undergoes adsorption onto the surface of the packing material, this effect may be at least partially compensated for [22, 32]. Both catalysts and packing material will interact with the plasma. Fully packing BaTiO3 and glass beads into the discharge gap have been shown to beneficially modify the discharge mode for the CO2 decomposition reaction when used as a packing material, resulting in an increase in the average electric field and mean electron energy due to the formation of surface discharges alongside the typical filamentary discharges formed in the absence of a packed bed [22]. The extent of filamentary discharge formation is reduced in a packed-bed reactor as this type of discharge can only be formed within the gaps between the pellets and between the pellets and reactor wall, rather than throughout the whole discharge area; instead, surface discharges form at pellet contact points and across the pellet surfaces [11, 22, 32]. The increase in electron energy and local electric field occurs at the bead contact points as the external electric field causes them to become polarized [19]. Breakdown occurs more readily as the electron temperature is higher than in an empty reactor due to greater acceleration of the electrons in the enhanced electric field [19, 22]. This leads to a more efficient usage of the applied electrical energy which contributes to maximizing the energy efficiency and conversion [19]. The presence of a catalyst can increase both energy efficiency and CO2 conversion simultaneously [33]. This is partly due to the electron temperature increasing when a catalyst is employed, even though the input power remains constant [33]. As mentioned above, polarization of the dielectric material occurs, enhancing the local electric field which increases the electron temperature [19, 22, 33]. At the contact points, the electron temperature has been shown to increase fourfold in comparison to the electron temperature in an empty reactor [33]. Consequently, there is a more efficient transference of energy from the applied electric power to the electrons in the form of heat [33]. As CO2 splitting occurs through excitation and dissociation and electron impact ionization when the electrons transfer energy to CO2, these processes are also more efficient, leading to an increase in energy efficiency in the presence of a catalyst [33]. Recent works have shown that the combination of DBD plasma with a photocatalyst (BaTiO3 and TiO2) using a partial catalyst packing configuration significantly enhanced the conversion of CO2 and energy efficiency by up to 250% at low temperatures (~150  C) compared to the plasma conversion of CO2 in the absence of a catalyst, as shown in Fig. 9.6 [29]. The presence of the catalyst pellets in the part of the discharge gap has been found to induce plasma physical effects, such as the enhanced local electric field by 10% due to the polarization of the catalytic materials which increases the electron temperature and produces more energetic electrons and reactive species. More importantly, this work has demonstrated that energetic electrons generated by the plasma have acted as the main driving force to activate both photocatalysts for CO2 conversion, making a major contribution to the enhanced CO2 conversion and energy efficiency, while the

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Fig. 9.6 Effect of photocatalysts (BaTiO3 and TiO2) on plasma conversion of CO2 at a SEI of 28 kJ/L [29]

UV emission generated by the plasma has played a minor role in the activation of these photocatalysts for CO2 conversion [29]. Thus, this exceptional synergistic effect resulting from the integration of DBD with photocatalysis for CO2 conversion at low temperatures can be attributed to both the physical effect induced by the presence of the catalyst in the discharge and the dominant photocatalytic surface reaction driven by energetic electrons from the CO2 discharge [29]. Furthermore, breakdown voltage is also affected by the packing material. In comparison to the breakdown voltage measured in a DBD reactor in the absence of a packing material (3.43 kV), the presence of glass beads has been shown to roughly halve this value (1.56 kV), while the use of BaTiO3 has been shown to decrease breakdown voltage to less than a third (1.03 kV) [22]. Additionally, the decrease in electron density caused by the use of a dielectric material promotes oxygen radical recombination which in turn impedes the recombination of CO and O to form CO2 [11, 34]. The reaction performance (CO2 conversion and energy efficiency) is therefore enhanced due to the physical changes resulting from the presence of the packing material, with CO2 conversion and energy efficiency increasing by up to 75% for a packed bed of BaTiO3 (albeit not simultaneously) in comparison to no packing material [22]. However, not all packing material gives the same effect; quartz wool is very porous and interacts strongly with plasma, leading to the formation of intense filamentary discharges [22]. A summary of the energy efficiency achieved in different plasma-catalytic systems can be seen in Table 9.1. Also included are the energy efficiencies achieved without the use of a catalyst, in order to ascertain the effect of the catalyst. Clearly, different types of plasma system result in varying energy efficiencies (defined as (Eq. 9.3)) (Table 9.1). Generally, gliding arc and microwave plasmas attain higher energy efficiencies than DBDs;

DBD Packed-bed DBD Packed-bed DBD Packed-bed DBD Packed-bed DBD

Plasma type DBD DBD DBD DBD GAD DBD DBD Corona Microwave

Packing materials/ catalysts – – – – – – – – NiO/TiO2 (Ar plasma treated) BaTiO3 CaO ZrO2 CaTiO3 BaTiO3 – 5.7 4.6 4.8 5.9

– 75.8 240 52.9 60.0

– 41.9 42.3 20.5 28.2

Energy efficiency (%) 2.8 1.9 2.0 1.7 14.1 12.6 1.6 1.7 –

Maximum CO2 conversion SEI CO2 conversion (kJ/L) (%) 120 27.2 229.0 34.0 297.6 46.6 60.0 18.2 15.4 17.4 22.2 24.3 240.0 30.0 80 10.9 – – 28 45.5 36 32.4 24.0

38.3 32.9 9.6 15.8 13.7

Maximum energy efficiency SEI CO2 conversion (kJ/L) (%) 24 20.0 4.3 3.1 1.6 2.3 20.0 9.6 9.8 15.2 11.7 16.0 45.1 14.0 5.2 3.1 30 41.3

Table 9.1 Comparison of CO2 conversion and energy efficiency using different atmospheric pressure plasma sources

16.6 7.1 9.6 6.1 7.1

Energy efficiency (%) 10.4 8.0 17.7 3.8 19.3 15.7 3.9 7.5 17.2

[29] [41] [19] [34] [22]

Ref [18] [12] [35] [36] [37] [38] [10] [39] [40]

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however, DBDs are more easily combined with a catalyst and are more suited to industrial-scale applications. The size and form of the packing material or catalyst can affect the reaction performance. If bead sizes are too small, the reaction performance can decrease in comparison to the reaction without packing. This is due to the decreased residence time, as well as the number of contact points being too great for the applied electrical energy to cause significant enhancement of the local electric field and the electron energy [19]. In one study that used a DBD reactor, CO2 conversion generally increased with an increase in ZrO2 bead size range (from 0.9–1 mm beads to 2–2.24 mm beads) at 80 W input power and various flow rates [19]. However, in another case, 0.18–0.25 mm beads of γ-Al2O3, MgO and CaO all resulted in a higher CO2 conversion in comparison to larger beads with size range 0.25–0.42 mm [11]. The decrease in conversion for the larger beads was attributed to an increased void fraction causing a drop in the electric field strength and discharge area. As can be seen from just these two results with conflicting conclusions, the interactions between plasma and packing material are far from simple and many factors will contribute to reaction performance, making it difficult to predict the outcome for a particular system. Another aspect which must be taken into account is the role the catalyst or packing material plays in the splitting of the CO2 molecule via adsorption of the molecule onto the beads. Two types of adsorption can occur: chemical (occurs with materials such as CaO and MgO) and physical (such as for γ-Al2O3). Chemisorption leads to a higher CO2 conversion as CO2 molecules adsorbed this way more readily decompose [11]. Chemisorption is affected by the acid-base properties of the packing material, with high basicity materials leading to a greater CO2 conversion as they aid adsorption. This is because highly basic metals are more easily reduced. The number of surface oxygen vacancies present in the catalyst is a highly important factor for determining CO2 conversion [29, 42], as dissociative electron attachment is facilitated by oxygen vacancies; hence a high number of vacancies can lead to an increase in CO2 conversion (Fig. 9.7). Recently, Mei and his co-workers have shown that the presence of oxygen vacancies on the surface of BaTiO3 and TiO2 photocatalysts contributes to the enhanced CO2 conversion in comparison to the plasma reaction without a catalyst [29]. They found that more oxygen vacancies were formed in BaTiO3 than in TiO2, resulting in the higher CO2 conversion using BaTiO3 in the plasma-catalytic conversion of CO2 [29]. Various pretreatments of catalysts can also be used for the synthesis of a catalyst with a large number of oxygen vacancies. One study used plasma pretreatment, in which three different gases (CO2, Ar and O2) were used to treat NiO/TiO2 catalysts [42]. Both O2 and CO2 pretreatments failed to result in a catalyst with high affinity for CO2 decomposition; however, the Ar pretreatment led to a catalyst that increased both the energy efficiency and CO2 conversion by a factor of 2 in comparison to the plasma-alone process. This difference in reaction performance between the catalysts prepared using different gases was attributed to the increase in the number of oxygen vacancies in the Ar-treated catalyst. This is because dissociative electron attachment occurs at the oxygen vacancy sites as CO2 is adsorbed more easily here than on

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a

b

Fig. 9.7 (a) Generation of oxygen vacancies (Vo) at the catalyst surface via bombardment with plasma-generated electrons; (b) CO2 dissociation mechanism via dissociative electron attachment at the catalyst surface

defect-free sites [42]. CO2 can then be dissociated to CO and O, through the transient CO2 ion, due to electrons from the plasma colliding with the molecule. The CO molecule is then desorbed from the active site and the O ion releases an electron as it fills the oxygen vacancy on the catalyst surface [42]. Furthermore, the oxygen vacancies are regenerated, preventing any decrease in catalytic activity [42]. This could be due to a gaseous oxygen atom in the plasma recombining with surface-adsorbed O2, according to (9.R6). As oxygen atoms in the plasma can also be in excited states, the recombination process may occur to an even greater extent due to this enhanced energy state [42]: Oads þ OðgÞ ! O2 ðgÞ

9.3

ð9:R6Þ

Plasma Dry Reforming of CH4 with CO2 CO2 þ CH4 ! 2CO þ 2H2

ð9:R7Þ

Dry reforming of methane with CO2 (9.R7) has the benefit of utilizing two greenhouse gases in the form of different sources (e.g. landfill gas, biogas and shale gas) in a single process. This process usually produces syngas, a mixture of hydrogen and carbon monoxide, alongside other valuable chemicals and fuels. Syngas is a vital chemical intermediate that can be used to produce a variety of chemicals and fuels, including via the Fischer-Tropsch process. Higher hydrocarbons, such as C2H2, C2H4, C2H6 and C3H8, can also form from the dry reforming

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reaction, although the concentration of these is always much smaller than that of syngas [43–45]. Very recently, Wang et al. have developed a water electrode DBD plasma reactor for the direct, one-step reforming of CH4 with CO2 into higher-value liquid fuels and chemicals (e.g. acetic acid, methanol, ethanol and formaldehyde) with high selectivity at room temperature (30  C) and atmospheric pressure (Fig. 9.8) [46]. The total selectivity to oxygenates was approximately 50–60%, with acetic acid being the major liquid product at 40.2% selectivity. Two possible reaction pathways could contribute to the formation of acetic acid in this process. CO can react with a CH3 radical to form an acetyl radical (CH3CO) with a low energy barrier of 28.77 kJ/mol, followed by recombination with OH to produce acetic acid with no energy barrier. Direct coupling of CH3 and carboxyl radicals (COOH) could also form acetic acid based on density functional theory (DFT) modeling. A few groups have also found the formation of trace oxygenates (e.g. alcohols and acids) as by-products of syngas production in plasma dry reforming of methane. In a DBD reactor, acetic, formic, butanoic and propanoic acids have all successfully been formed, along with methanol and ethanol [43]. There are a number of pathways through which formic acid and propanoic acid could form. The most likely pathway is the addition of CO to an ethyl radical, although an ethyl radical may also couple with a carboxyl radical; furthermore, the carboxyl radical could couple with a hydrogen radical to form formic acid. The carboxyl radicals required for acid formation are thought to result from the addition of CO and OH [43]. Carbon nanomaterials are also possible by-products of the dry reforming reaction (Fig. 9.9). Multiwall carbon nanotubes and spherical carbon nanoparticles have been formed in a gliding arc reactor [4]. These are important by-products as carbon

Fig. 9.8 Direct and indirect dry reforming of methane to liquid fuels and chemicals [46]

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Fig. 9.9 TEM images of carbon nanomaterials formed in the plasma dry reforming of CH4 and CO2 using a gliding arc plasma: (a) spherical carbon nanoparticles and (b) carbon nanotubes [4]

nanotubes have a variety of applications; this could prove a sustainable and energyefficient method for their production if further research is focused in this area [4].

9.3.1

Influence of Process Parameters

The influence of a wide range of operating parameters on the energy efficiency and conversions of reactants has been investigated using different plasma systems. The reactant conversion and the H2/CO molar ratio, along with the product yields and selectivities, are affected by the molar ratio of CO2/CH4 in the feed [47, 48]. An increase in CO2 content in the feed leads to a rise in both energy efficiency and total conversion [5, 49, 50]. This is because CO2 and CH4 decomposition occur via electron impact dissociation, forming O and H atoms, respectively. These atoms in turn react with each other to form OH, thus limiting the recombination of CH3 and H and increasing CH4 conversion [5].

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One important factor that determines reaction performance is the number of micro-discharge filaments that come into contact with the gas molecules in the DBD, with a greater number of lower-energy filaments resulting in an increase in energy efficiency and conversion [5]. It has been reported that duty cycle affects CO2 and CH4 conversion in a DBD, with conversion increasing with duty cycle, when a sinusoidal voltage with square wave modulation is applied [43]. Pulsed plasma has been shown to effectively enhance the performance of the dry reforming process, with an increase in pulse frequency or applied peak voltage leading to higher total conversion [48, 51]. An increase in plasma power results in higher total conversion as a greater number of higher-energy electrons are formed, which can go on to initiate reactions [5]. However, input power can also affect product distribution, with a greater concentration of carbon powder and water forming at higher power. The use of a longer residence time can also increase total conversion, as the SEI is increased; however, it bears almost no impact on CO and H2 selectivities [5, 48]. The SEI term incorporates both power and residence time, and if both parameters are adjusted but the SEI remains constant, the change in total conversion will be negligible [5]. However, an increase in SEI leads to higher CO2 and CH4 conversions in a variety of plasmas (gliding arc, DBD, corona), but has the opposite effect on the energy efficiency, resulting in a trade-off between the two [5]. Optical emission spectroscopic diagnostics has been used to understand the formation of a wide range of reactive species generated in the reforming process [27], while plasma chemical kinetic modeling has been used to understand the underlying plasma chemistry and reaction pathways of the dry reforming process [45]. The latter model demonstrates how selectivity towards different products can be achieved through manipulation of the residence time due to the spatially averaged densities of some molecules continually increasing with residence time, while others peak at a certain value [45]. In order to maximize reactant conversion and energy efficiency, the process parameters must be optimized. This is a difficult task as each plasma system will have a different set of optimal parameters and numerous experiments must be carried out to realize these. Alternatively, a modeling approach can be used. Some models have now shown good agreement with experimental results, even though they are much simplified versions of the actual plasma chemistry. As mentioned previously, a trade-off exists between energy efficiency and reactant conversion; hence a middle ground must be found that results in a viable process.

9.3.2

Catalytic Reforming Versus Plasma-Catalytic Reforming

As shown by 9.R7, dry reforming is a highly endothermic process. As such, high temperatures are required (>1000  C) in the thermal process to overcome the stability of both the CO2 and CH4 molecules. A catalyst can be used to convert the

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reactants at lower temperatures; however, reactions temperatures must still be above 700  C [47, 52]. Carbon deposition can also occur on the catalyst surface, leading to catalyst deactivation [47, 52]. Both these issues incur cost, as the energy input must be high and catalysts must be replaced periodically. In order to overcome these issues, catalysts can be combined with NTP. In the plasma-catalytic system, the benefits of using NTP which can overcome the high stability of the CO2 and CH4 molecules at low temperature and atmospheric pressure, along with those of catalysis (reduction in activation barrier, increased selectivity and conversion), can be realized simultaneously. This can increase reaction performance [53, 54], as well as reducing costs. Furthermore, interactions occur between the catalyst and the plasma which can lead to a synergistic effect in terms of conversion and energy efficiency [47, 55]. However, coking can still be an issue in plasma-catalytic systems, although this may be dampened through removal by excited hydrogen species [53]. One method to overcome this is to flow pure CO2 through the reactor to remove the deposited carbon [53]. The oxidation of carbon by CO2 occurs much faster in a DBD reactor than a thermal one [53]. Thermal-catalytic and plasma-catalytic dry reforming differ in the reagent conversion ratio. Conversions of both CO2 and CH4 should be equal due to the stoichiometry of the reaction; however, as a result of the reverse water-gas shift reaction, in the thermal process, a higher conversion of CO2 occurs in comparison to CH4. This differs to the plasma reaction, in which the CH4 conversion is higher. This can be attributed to the prevalence of gas-phase reactions that lead to the dissociation of the CH4 molecule (9.R8, 9.R9, and 9.R10), as well as the production of CO2 via 9.R11 [47]: CH4 þ e ! CH3 þ H þ e

ð9:R8Þ

CH4 þ e ! CH2 þ H2 þ e

ð9:R9Þ

CH4 þ e ! CH þ H þ H2 þ e

ð9:R10Þ

CO2 þ þ CH4 ! CH4 þ þ CO2

ð9:R11Þ

As mentioned above, in the plasma-catalytic dry reforming reaction, plasma reactions occur in the gas phase to dissociate CH4 (9.R8, 9.R9, 9.R10, and 9.R11) and CO2 (9.R12) [51]: CO2 þ e ! CO þ O þ e

ð9:R12Þ

Active species created in the plasma can also adsorb onto the catalyst surface, from where they can form CO and H2 products [55]: CHx þ □ ! ðCHx Þad

ð9:R13Þ

H þ □ ! Had

ð9:R14Þ

O þ □ ! Oad

ð9:R15Þ

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ðCHx Þad þ Oad ! COad þ xHad

ð9:R16Þ

COad ! CO þ ◻

ð9:R17Þ

2Had ! H2 þ □

ð9:R18Þ

where □ is an empty adsorption site present on the catalyst surface. The simultaneous occurrence of both plasma and surface reactions in plasma-catalytic systems at low temperature can lead to synergy in terms of product selectivities and reactant conversions, which cannot occur in the catalytic process at low temperature due to insufficient energy input. Desorption of the species on the surface of the catalyst can also occur more readily in the plasma process as the plasma can affect the catalyst properties [55]. Thus, the plasma-catalytic process is more beneficial due to increased CO2 and CH4 conversion and product selectivities and a reduction in coking at lower energy input than the thermal catalytic process.

9.3.3

Influence of Catalyst

In the thermal catalytic dry reforming process, supported metal catalysts have been used widely, with non-noble metals being prevalent due to their low cost and wide availability. Catalysts with high activity for the thermal catalytic process have been used as the starting point for the plasma-catalytic process. Supported metal catalysts with a core and shell structure have been investigated as this allows the active sites to be uniformly distributed, while deactivation due to carbon deposition and sintering is kept to a minimum as a result of the strong interactions between the core and shell [56]. Zeolite 3A [57], NaX and NaY [58, 59], Ni/γ-Al2O3 [27, 30, 47, 60–62], Co/γ-Al2O3 [47], Mn/γ-Al2O3 [47], Ag/Al2O3 [63], Pd/Al2O3 [63, 64], Cu/Al2O3 [47, 64], Fe/Al2O3 [65], La2O3/γ-Al2O3 [66], LaNiO3 [67], Cu-Ni/Al2O3 [55] and LaNiO3@SiO2 [56, 68] catalysts have all been tested in the plasma-catalytic process, with Ni/γ-Al2O3 being the most commonly used. More recently, K-, Mg- and Ce-promoted Ni/γ-Al2O3 catalysts have also been evaluated in plasma-catalytic dry reforming of methane and CO2 [69]. However, the scope of catalysts available that are active for the thermal process has only just been touched upon for plasma dry reforming. In the plasma-catalytic system, the catalyst structure can be influenced by the plasma, while the presence of the catalyst can affect the discharge properties [70, 71]. As a result of the interactions between the plasma and the catalyst, a synergistic effect can result, whereby the reaction performance is greater than the sum of the plasma-alone and the purely catalytic processes [27, 55]. The formation of radicals in the plasma can change the catalysts reaction mechanism as these species are adsorbed onto the catalyst surface, while adsorbed vibrational excited species can facilitate CO2 and CH4 dissociation through dissociative adsorption due to their high internal energies [55, 70]. It may also be possible that the catalytic activity is improved due to the charged particles on the catalyst surface and the

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applied voltage acting similarly to the electrochemical promotion of catalysis [70]. Other plasma influences include changing the lattice structure of the catalyst due to the transference of thermal energy from ions and electrons causing thermal perturbation which can increase coking resistance and catalyst activity, as well as structural changes due to particle bombardments which lead to changes in catalyst pores, active metals and promoters [70]. One such change that can occur is in the crystallinity of the active metal and support due to variation in valence state, resulting in an increase in oxygen affinity which leads to greater conversion of CH4 [70]. The changes discussed this far are a result of the plasma changing the properties of the catalyst. As mentioned above, the catalyst can also affect the plasma properties [53, 70]. Catalysts change the discharge mode to a combination of micro-discharge and surface discharge [71]. An increase in the local electron density due to energy concentrating in the gaps between the catalyst pellets results in an increase in reactions occurring in both the plasma and at the catalyst surface as the electric field is enhanced [70]. The dielectric constant of the catalyst also affects the plasma, with a high dielectric constant catalyst resulting in an increase in the plasma electric field [53, 70, 71]. High dielectric catalysts, such as ferroelectrics, can increase the production of syngas in the dry reforming reaction [70]. The catalyst properties, such as dielectric constant, and geometry are therefore highly important in determining reaction performance [54, 72]; thus, the reaction performance can be optimized through selection of catalyst. Packing geometry can also influence the interactions between the plasma and the catalyst; in a DBD reactor, partially packing a Ni/γ-Al2O3 catalyst into the discharge gap results in an enhancement in reaction performance in comparison to a fully packed reactor [27, 30]. This is because the discharge in the partially packed reactor retains the strong filamentary discharge, whereas the reduction in discharge volume in the fully packed reactor changes the discharge mode to surface discharge and spatially limited micro-discharge [27]. In a DBD reactor, the use of a Ni/γ-Al2O3 catalyst has been shown to enhance the conversion of CH4, along with the yield of CO and H2, in comparison to the plasmaalone process; however, the CO2 conversion decreased slightly upon addition of the catalyst [47]. This Ni/γ-Al2O3 catalyst resulted in higher H2 and CO yields and CH4 conversion than the Co/γ-Al2O3, Cu/γ-Al2O3 and Mn/γ-Al2O3 catalysts that were also tested in this reactor, with a maximum CH4 conversion of 19.6% being achieved at a flow rate of 50 ml/min and 7.5 W discharge power [47]. Although the energy efficiency of the plasma reaction is not always increased by the addition of a catalyst, such as is the case when Co/γ-Al2O3 or Cu/γ-Al2O3 catalysts are used, both Ni/γ-Al2O3 and Mn/γ-Al2O3 catalysts were found to enhance the energy efficiency [47]. Energy efficiency is higher in gliding arc discharge in comparison to other types of discharge, and catalysts can increase this still further [54]. The use of a NiO/Al2O3 catalyst, placed in the afterglow of the discharge in a gliding arc reactor, was found to increase energy efficiency by over 20% in comparison to that achieved using plasma only [54]. H2 yield, along with CO2 and CH4 conversions, was also increased. The concentration of active metal was found to influence reaction

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performance, as a 33wt% NiO/Al2O3 catalyst resulted in a decrease in reaction performance in comparison to an 18wt% NiO/Al2O3 catalyst, while a smaller catalyst diameter was found to be beneficial [54]. The addition of dopants and use of bimetallic catalysts have also been studied. Zhang et al. investigated the effect of varying the concentrations of Cu and Ni in various Cu-Ni/γ-Al2O3 catalysts and found that the 12 wt% Cu-12 wt % Ni/γ-Al2O3 catalyst gave the optimum results for both CH4 and CO2 conversion (Fig. 9.10) [55]. This catalyst also achieved the maximum selectivity to CO of 75%. However, this selectivity was also achieved when using the 5 wt% Ni-12 wt% Cu/γ-Al2O3 catalyst, whereas the maximum selectivity to H2 was achieved with 16 wt% Ni-12 wt % Cu/γ-Al2O3 and 20 wt% Ni-12 wt% Cu/γ-Al2O3 catalysts [55]. Another factor which must be taken into consideration is the catalyst support, as the support, along with the interactions between it and the active metal, can affect the reaction performance. A study completed by Mei et al. investigated the use of a Ni catalyst supported on γ-Al2O3, TiO2, MgO and SiO2 [73]. The results of this experiment concluded that the γ-Al2O3 support was most beneficial on the reaction performance, giving the highest CO2 (26.2%) and CH4 (44.1%) conversions, as well as the maximum achieved energy efficiency and highest yields of CO and H2. This was attributed to the increased reducibility of the Ni/γ-Al2O3 catalyst and the number of stronger basic sites present at its surface (which facilitate CO2 chemisorption and activation), along with its higher specific surface area and greater dispersion of smaller NiO particles [73]. Carbon deposition also occurred to a lower extent on this catalyst, as the increase in CO2 chemisorption and activation may have resulted in adsorbed CO2 undergoing gasification by surface-adsorbed oxygen [73]. Weaker interactions between the catalyst and support are favorable as this increases the reducibility of the catalyst, increasing its activity [27].

Fig. 9.10 Conversion of CH4 and CO2 in the dry reforming reaction using three different processes (plasma only, catalysis only and plasma catalysis) [55]

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Plasma CO2 Hydrogenation

A particularly significant route being developed for CO2 conversion is CO2 hydrogenation, which has a lower thermodynamic limitation compared to direct CO2 decomposition and dry reforming of methane. Carbon dioxide can be hydrogenated in plasma via reaction with hydrogen at atmospheric pressure, thus avoiding the use of high pressure required by conventional thermal catalytic processes. CO2 methanation (9.R20) and the reverse water-gas shift (RWGS) reaction (9.R19) prevail when hydrogen is reacted with CO2 in the plasma process [74]; however, it is also possible to produce higher-value chemicals and fuels such as methanol and ethanol [75]. The main barrier to this process is the source of hydrogen. In order for this process to be both economically viable and sustainable, hydrogen must be produced using a low-cost, environmentally friendly and sustainable process. Currently, coal gasification and steam reforming of methane dominate the production pathways of H2, leading to the emission of CO2 [76]. Due to this, CO2 conversion using hydrogen must convert a greater amount of CO2 than hydrogen production pathways generate [76]. In thermal catalysis, the production of methane from H2 and CO2 is not considered a viable method of fuel production due to the low energy per unit volume and high H2 consumption [77]. As plasma processes have the potential to convert large amounts of CO2 at high energy efficiencies, interest is increasing into the development of these systems. In comparison to thermal CO2 hydrogenation which requires high temperature and high pressure (30–300 bar), plasma systems operate at room temperature and pressure, hence increasing their viability. If plasma systems can be combined with a sustainable source of hydrogen, such as from water splitting using renewable energy, or indeed be used to split the hydrogen source in situ, this could prove a vital pathway for CO2 mitigation.

9.4.1

CO2 Hydrogenation to CO CO2 þ H2 ! CO þ H2 O

ð9:R19Þ

The conversion of CO2 in this reaction has been found to rise as the H2 content in the feed is increased. A higher ratio (H2/CO2) can also increase CO selectivity and yield, with a ratio of 4:1 resulting in a slight increase in CO selectivity and a threefold increase in CO yield in comparison to a feed ratio of 1:1 in a DBD [74]. Furthermore, this increase in feed ratio also results in a rise in CO production efficiency. The selectivity to CO has been found to increase with a rise in total flow rate [78]. This is most likely due to the decreased residence time associated with an increase in flow rate resulting in the recombination of CO and O being suppressed, along with the further hydrogenation of CO to form hydrocarbons. This hypothesis is

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supported by the decrease in selectivity to CH4 that occurs as the total flow rate is raised, as detailed below. In order to enhance the production of CO, a catalyst can be added into the reactor. In a DBD reactor, it was found that the addition of a Mn/γ-Al2O3 catalyst leads to an increase (of 114%) in CO yield in comparison to the plasma-alone process, as well as an increase (of 116%) in CO production efficiency [74].

9.4.2

CO2 Hydrogenation to CH4 CO2 þ 4H2 ! CH4 þ 2H2 O

ð9:R20Þ

In this process, a higher H2 content in the feed in comparison to CO2 is desirable as this increases the conversion of CO2. This has been determined both experimentally [74, 79] and through the use of a 1D fluid model [76]. A 3:1 ratio (H2/CO2) is optimal for enhancing CH4 yield [74]. Optimizing the total flow rate can also maximize the CH4 selectivity and CO2 conversion. A very low total flow rate can lead to reverse reactions occurring, reforming CO from CH4 according to 9.R21, due to the longer residence time increasing the interactions between the CO2 hydrogenation products and the excited species in the plasma [78]: CH4 þ H2 O ! CO þ 3H2

ð9:R21Þ

As the total flow rate is increased, the rate of formation of CH4 becomes larger than the rate of the reverse reaction, leading to an increase in CH4. At high total flow rates however, the residence time is too low for reagent gases to interact with excited plasma species, decreasing the overall production of CH4 as both the forward and reverse reactions occur to a lesser extent [78]. For DBD plasmas, the use of an alumina reactor instead of quartz is beneficial on reaction performance due to the enhanced relative dielectric permittivity coefficient of alumina [79]. Addition of a magnetic field can enhance CO2 conversion, increasing the CH4 selectivity by over 10% at a discharge power of 30 W while also tripling the energy efficiency of the process [80]. This study however employed low pressure (200 Pa), reducing simplicity of design and requiring extra energy input, thus detracting from the benefits of NTP. The function of the magnetic field is to prevent electrons from diffusing to the downstream region. In the downstream region, CH4 is produced but also decomposed through electron impact dissociation (Eq. 9.4), leading to a decrease in the yield of CH4 [80]. Magnetized electrons cannot travel out of the magnetic field; hence the downstream recombination reaction is suppressed, while CH4 production can still proceed via reactions involving neutral radicals as these are not magnetized. The magnetic field also increases the electron density, due to

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confinement of the electrons in the magnetic field, which leads to an increase in the CO2 decomposition reaction as well as the energy efficiency [80]. An increase in power input generally results in a higher selectivity to CH4 due to the increase in power density this results in [79, 80]. However, it has been found that at high-power input (>160 W), energy is transferred to the electrodes through heating rather than being used for plasma production, resulting in no further increase in CH4 selectivity [80]. A rise in voltage also causes an increase in plasma density, which again leads to an increase in CH4 selectivity. In radio frequency (RF) discharge, the relationship between CH4 concentration and voltage can be expressed as [78]: ½CH4  / ðV ÞN ðN  2Þ

ð9:4Þ

Hence at higher voltages, the production efficiency of CH4 increases. A study into the CH4 production dependence on the repetition frequency of a low-pressure RF discharge has shown that CH4 yield increases with repetition frequency [78]. The production of CO increases linearly with repetition frequency (up to its saturation point) due to an increase in the number of electrons as CO is formed via electron impact dissociation of CO2 [78]. CH4 is formed via reaction between CO and H; hence there is a power law relationship between the formation of CH4 and the repetition frequency (and number of electrons) [78]. A smaller discharge gap is beneficial on the CO2 conversion and CH4 selectivity due to the rise in input power density caused by enhancement in the electric field [80]. In fact, a smaller discharge gap can achieve the same CH4 selectivity at a lower input power than when using a larger discharge gap [80]. A reduction in discharge gap can also increase the production efficiency of the system. It must be noted here that for the case of [80], a decrease in discharge gap also resulted in an increase in magnetic field. As expected, an increase in power input results in a decreased energy efficiency [80]. However high-power inputs give rise to larger conversions. Catalysts have therefore been employed as a method to combat this trade-off. The use of Mn/γ-Al2O3 and Cu/γ-Al2O3 catalysts in a coaxial DBD reactor has been found to increase CO2 conversion as well as the energy efficiency of both CH4 and CO production in comparison to the plasma-alone process [74]. The Cu/γ-Al2O3 catalyst was found most beneficial for the production of CH4 as this catalyst achieved the highest yield and selectivity to CH4, while the maximum CO2 conversion was achieved using the Mn/γ-Al2O3 catalyst. However, the Mn/γ-Al2O3 catalyst resulted in a decrease in CH4 selectivity in comparison to the process in the absence of a catalyst. The decrease in CO2 conversion attained when using the Cu/γ-Al2O3 catalyst in comparison to the Mn/γ-Al2O3 catalyst may be attributed to the increased prevalence of the water gas shift reaction in the presence of this catalyst as Cu catalysts are often used for catalyzing this reaction; hence the apparent CO2 conversion will be reduced [74]. It is therefore important to select a catalyst that will suppress the water gas shift reaction and simultaneously increase the CO2 conversion and the selectivity to CH4.

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The combination of plasma and catalysis allows the CO2 methanation reaction to occur at much lower temperatures than those required in the purely catalytic process [81]. Below 250  C, negligible CO2 conversion occurs for the catalytic process; however, when combined with plasma, the CO2 conversion reaches 80%, with 90% selectivity to CH4, for the addition of a Ce-Zr supported Ni catalyst in a DBD reactor [81]. This is due to the creation of excited species in the plasma, which generate new pathways for CO2 dissociation; hence the reaction is not limited by the rate of CO2 dissociation at the catalyst surface as it is in the purely catalytic process [82]. The use of nickel-containing hydrotalcite catalysts has also shown promise in the plasmacatalytic CO2 methanation reaction, with a CO2 conversion of 80% and selectivity to CH4 of nearly 100% having been achieved in a DBD reactor [83]. It is thought that the high number of low- and medium-strength basic sites is responsible for the high activity of this catalyst, as when promoted with metals containing high-strength basic sites (Ce and Zr), the conversion and yield decrease and no other important morphological changes could be identified [83].

9.4.3

Pathways of CO and CH4 Formation

In order for methane to form, CO2 is first dissociated to CO. Reaction between CO and H2 can then occur to form CH4. However, the oxygen radical produced in the dissociation of CO2 will compete to react with hydrogen, forming water [84]. The dominating pathway for production of CO occurs via electron impact dissociation of CO2 (9.R12). At low CO2 concentrations in the feed, electron impact dissociation is also the main pathway for the dissociation of H2 (9.R22); however, at high CO2 concentrations, H2 is mostly consumed through reaction with H2O+ and H3O+ [76]: H2 þ e ! H2  ! H þ H

ð9:R22Þ

Dissociation of CO can also occur; however, this reaction is highly endothermic; hence it occurs to a much lesser extent [78]. The net loss rate of CO2 remains constant at all inlet concentrations, but the net loss rate of H2 varies. At high H2 concentrations, the net loss rate is high as there is more H2 in the feed; as the H2 concentration is decreased, the net loss rate follows the same trend [76]. The net loss rate of H2 is higher than that of CO2, with this effect being more pronounced at high H2 concentrations. Downstream of the reactor, electron energies have decreased and are usually insufficient for dissociation reactions. Recombination reactions prevail (9.R23 and 9.R24) as these exothermic reactions only require low-energy electrons [78]: CO þ 3H2 ! CH4 þ H2 O

ð9:R23Þ

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CO þ 2H2 ! CH3 OH

ð9:R24Þ

However, the dissociation of CH4 can occur via electron impact dissociation (9. R25) [80], reducing the obtained yield: CH4 þ e ! CHx þ ð4  xÞH þ eðx ¼ 1, 2, 3Þ

ð9:R25Þ

The water gas shift (WGS) reaction (9.R26) can also occur, decreasing the apparent CO2 and H2 conversions [74]: CO þ H2 O ! CO2 þ H2

ð9:R26Þ

In this simplified summary of reaction pathways, it can therefore be seen that the formation of CH4 occurs via CO formation, which forms as a result of CO2 dissociation. Methanol is also a possible product. Unwanted reactions also occur in the plasma system, such as the water gas shift reaction and the dissociation of CH4; hence these reactions must be suppressed to ensure high yields of CO and CH4.

9.4.4

CO2 Hydrogenation to Liquid Fuels

CO2 hydrogenation to liquid fuels (e.g. methanol, ethanol and dimethyl ether (DME)) is one of the most attractive routes for CO2 conversion and utilization (Fig. 9.11). Significant efforts have been concentrated on CO2 hydrogenation to methanol (9.R27) using heterogeneous catalysis at high pressures [85]. CH3OH is a valuable fuel substitute and additive and is also a key feedstock for the synthesis of other higher-value chemicals. In addition, methanol is considered a promising hydrogen carrier, suitable for storage and transportation [85]: CO2 þ 3H2 ! CH3 OH þ H2 O

ð9:R27Þ

Cu-based catalysts have attracted considerable interest for catalytic hydrogenation of CO2 to methanol, owing to the excellent activity of metallic Cu for this reaction. Extensive efforts have also been devoted to modifying the structure of Cu-based catalysts using various supports (Al2O3, ZnO, ZrO2, SiO2, Nb2O5, Mo2C and carbon materials, etc.), promoters (Zn, Zr, Ce, Ga, Si, V, K, Ti, B, F and Cr) and preparation methods [85–87]. Up until now, very limited research has concentrated on CO2 hydrogenation using nonthermal plasmas, either with or without a catalyst [88–91]. The majority of this research reports CO as the dominant chemical, with CH4 formed as a minor product and no CH3OH detected [80–82]. In the late 1990s, Eliasson and co-workers investigated CO2 hydrogenation to CH3OH using a DBD plasma reactor [92]. However, only trace amounts of CH3OH were produced, with a maximum CH3OH yield

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Fig. 9.11 Scheme of CO2 hydrogenation to methanol [75]

of 0.2% obtained at atmospheric pressure (1 bar), a relatively high plasma power of 400 W, a total flow rate of 250 ml/min and a H2/CO2 molar ratio of 3:1. They also found that packing a Cu/ZnO/Al2O3 catalyst (a commercial methanol synthesis catalyst) in the discharge increased the methanol yield (from 0.1 to 1.0%), methanol selectivity (from 0.4 to 10.0%) and CO2 conversion (from 12.4% to 14.0%) at a higher pressure (8 bar) under similar operating conditions [92]. However, the methanol yield and selectivity were still significantly lower than those reported in catalytic CO2 hydrogenation processes. The formation of trace CH3OH in plasma CO2 reduction was also reported using a radio frequency impulse discharge at low pressures (1–10 torr) [78]. Very recently, Wang et al. developed a specially designed water electrode DBD reactor for the highly selective hydrogenation of CO2 to methanol at room temperature (30  C) and atmospheric pressure [75]. They found that the methanol production was strongly dependent on the structure of the DBD reactor; the DBD reactor with a special water electrode design and a single dielectric showed the highest reaction performance in terms of the conversion of CO2 and methanol yield (Fig. 9.12) [75]. The combination of the plasma with Cu/γ-Al2O3 or Pt/γ-Al2O3 catalyst significantly enhanced the CO2 conversion and methanol yield compared to the plasma hydrogenation of CO2 without a catalyst. The maximum methanol yield of 11.3% and methanol selectivity of 53.7% were achieved over the Cu/γ-Al2O3 catalyst with a CO2 conversion of 21.2% in the plasma-catalytic process, while no reaction occurred at ambient conditions without using plasma [75]. Compared to catalytic CO2 hydrogenation to methanol, which has been carried out using a wide range of catalysts, very limited catalysts that are active for thermal catalytic process have been examined for plasma hydrogenation of CO2. In addition, the production of dimethyl ether from plasma CO2 hydrogenation was reported using an atmospheric pressure surface discharge with a CO2 conversion of 15% and a H2/CO2 molar ratio of 1:1 [91].

298 Fig. 9.12 Influence of reactor structure (reactor I, II and III) on plasma hydrogenation of CO2 process: (a) concentration of oxygenates; (b) selectivity of gas and liquid products; (c) methanol yield and CO2 conversion (reaction pressure 1 atm, H2/CO2 molar ratio 3:1) [75]

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299

Plasma CO2 Conversion with Water

Carbon dioxide can be hydrogenated with water to produce syngas [93, 94]. Limited research exists into this method of splitting CO2. A higher H2O content in the feed appears to be beneficial on the production of H2, although the study citing this only uses ratios between 10:50 and 50:50 (H2O/CO2); hence it is unclear if further increasing the H2O content will continue the trend [93]. The opposite is true for the production of CO [93]. SEI can affect product yields, with maximum H2 yields occurring at low SEI [93]. A rise in the feed flow rate results in H2 yields decreasing; this is expected due to the decrease in residence time and the relationship between SEI and flow rate discussed previously. The reduction in H2 yield due to high SEI and/or increased flow rate can be attributed to the occurrence of the reverse water gas shift reaction, as the yield of CO remains constant [93]: Syngas is not the only possible product of plasma CO2 hydrogenation with water: methane can also be produced [94, 95]. Although the current obtainable methane concentration is low (ppm) and reported energy efficiencies are well below feasible, the proof of concept is important. Methane production from CO2 and water using NTP could potentially provide a one-step process for creating a useful energy source from a sustainable source of hydrogen. The addition of a catalyst to this reaction can remarkably increase the yield of methane, as dissociative adsorption of H2, CO (formed via plasma gas phase reactions) and CO2 occurs at the catalyst surface, enabling the hydrogenation of carbon species via this mechanism as opposed to through plasma gas-phase reactions alone [95]. The use of a NiO/Al2O3 catalyst is beneficial for the production of methane as this catalyst facilitates the hydrogenation of CO [94]. Furthermore, the use of a reduced Ni catalyst can facilitate the production of carbon nanofibers through plasma-assisted chemical vapor decomposition of methane (Fig. 9.13) [94]. The reduction of CO2 using water is possible using electrochemical processes [96] as well as through photoreduction [97]. These processes produce methanol, an important chemical intermediate [96]. To the best of the authors’ knowledge, this reaction has not been carried out using plasma processes; however, the production of methanol has been successful in a DBD (albeit in very small quantities) when using hydrogen in the feed [92]. The addition of a CuO/ZnO/Al2O3 catalyst has also been shown to increase the methanol yield by a factor of 10 [92]. As H2O can be successfully split into H2 in plasma, the production of methanol from CO2 and H2O is theoretically possible. The main competing reaction for the formation of methanol is the production of methane [92].

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Fig. 9.13 Formation of CNF in the plasma conversion of CO2 with water over Ni/Al2O3 catalyst [94]

9.6

Summary and Future Outlook

Plasma-based technologies for the conversion of CO2 into value-added fuels and chemicals show great potential due to the ability of nonthermal plasma to break bonds in the highly stable CO2 molecule while operating at room temperature and pressure. Plasma systems therefore have an advantage over thermal processes, which require high-temperature inputs; hence plasma conversion of CO2 could prove much more feasible on an industrial scale. However, a trade-off between energy efficiency and CO2 conversion currently exists in the plasma process as conversion increases when energy input is raised, which also causes a decrease in energy efficiency. Initial research has shown that this problem can be overcome by modifying the plasma system, such as by combining the plasma discharge with a catalyst; however, further research is required to promote the simultaneous increase of energy efficiency and conversion. Once plasma processes can concurrently operate at high conversion and energy efficiency, they will become a front runner in green technologies for the conversion of carbon dioxide. Plasma chemistry is highly complex, and although much research is being conducted into plasma modeling, the models being used are greatly simplified

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versions of the reactions taking place [16, 93, 98]. Newer models can also leave their predecessors redundant; therefore, further study is required to produce comprehensive models for a variety of plasma processes [99]. When it comes to plasma catalysis, the chemistry becomes even more complex due to the interactions occurring between the plasma and catalyst. The number of different catalysts that can be employed in plasma processes, along with variations in catalyst preparation method, loading amount, pretreatment, etc., makes it tricky to use a ‘one-model-fits-all’ approach. If a greater understanding of plasma reactions and the interactions between plasma and catalyst can be realized, a more comprehensive model could be produced. In situ experimental techniques, such as in situ infrared spectroscopy, can lead to a greater understanding of plasma-assisted surface reactions, for example, plasma adsorption or desorption on the surface of the catalyst. Using techniques such as this could help to create such a model, drastically reducing the time required to optimize a process and leading to selection of the optimum catalyst without the need for numerous laborious experiments. The potential exists to produce more complex carbon-based liquid products using plasma. A variety of liquids can currently be produced in small quantities, such as formaldehyde, acetic acid and methanol, as well as ethylene and C4 hydrocarbons [100, 101]. However, the selectivity to many of these products still needs to be improved for the process to be viable. The selection of an appropriate catalyst that can increase the selectivity to the required product is therefore required for progress to be made in this area. Thermal catalytic techniques are currently used to produce liquid hydrocarbons such as DME from carbon dioxide and hydrogen. For the production of DME, high temperatures (240–270  C) and pressures (3 MPa) are required [102–104]. If NTP at atmospheric pressure can be used instead, the energy input can be drastically reduced. In order to produce hydrocarbons directly from CO2, bifunctional catalysts are required [105]. Novel catalysts and new reactor setups may help offset the need for high pressures in a plasma reactor. Much more research is needed to create plasma processes that produce liquid hydrocarbons currently only produced by other (non-plasma) techniques, but if successful these processes could transform the chemical and energy industries. The scope and potential of plasma processes for the utilization of CO2 are therefore vast. These processes reduce the concentration of CO2 in our atmosphere and allow the chemical storage of energy which can be transferred to the system from renewable energy sources at peak times. As well as producing fuels, valuable chemicals can also be formed. A greater understanding of the plasma chemistry, both through modeling of plasma and coupling with other techniques such as catalysis, as well as further insight into synthesizing a catalyst which will create synergy when combined with plasma [106], will allow this field to expand. Alongside this, further research into the conversion of CO2 in feed gases mixed with other gases from industrial waste streams could also be beneficial for creating large-scale plasma processes for industrial applications [24].

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Acknowledgments The authors acknowledge financial support from the UK EPSRC SUPERGEN Hydrogen & Fuel Cell (H2FC) Hub (EP/J016454/1), EPSRC SUPERGEN Bioenergy Challenge II Programme (EP/M013162/1), and EPSRC Impact Acceleration Account (IAA). We acknowledge the funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Action (Grant Number 823745).

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Chapter 10

Plasma-Catalytic Reforming of Alcohols Dae Hoon Lee

10.1

Introduction

Methanol and ethanol are one of the most important basic materials for chemical industries. Methanol is used as a raw material in a wide range of fields such as synthetic detergents, photosensitive materials, synthetic resins, ethers, esters, dyes, and adhesives in the chemical industry and as a test and disinfectant in the medical field [1–4]. In recent years, as environmental problems and a lack of petroleum resources are expected, the need for various types of nontraditional energy sources such as renewable energy is increasing. Since a typical alternative energy source is biomass-derived fuels, which consist of alcohols, attempts have been made to utilize alcohol, especially methanol, for various energy sources and as chemical raw materials in this background [5–7]. This movement is largely driven by (1) the use of alcohol itself as a fuel, (2) the production of hydrocarbons such as olefins and paraffins through chemical synthesis and the production of synthetic oils therefrom, (3) and the production of hydrogen via reforming. A typical case of using the alcohol itself is as a fuel to directly produce electricity, such as in a DMFC (direct methanol fuel cell) of which the principles of operation are schematically shown in Fig. 10.1 [8]. In recent years, biofuels have been used for transportation in the sense of utilizing renewable energy. Among possible biofuel resources, bioethanol extracted from

D. H. Lee (*) Plasma Engineering Laboratory, Korea Institute of Machinery and Materials, Yuseong-Gu, Daejeon, South Korea Department of Environment & Energy Engineering, University of Science and Technology, Yuseong-Gu, Daejeon, South Korea e-mail: [email protected] © Springer Nature Switzerland AG 2019 X. Tu et al. (eds.), Plasma Catalysis, Springer Series on Atomic, Optical, and Plasma Physics 106, https://doi.org/10.1007/978-3-030-05189-1_10

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Fig. 10.1 Schematic principle of a DMFC in operation [8]

biomass fermentation is the most widely used among them [9, 10]. In the United States, 96% of renewable fuel additives are bioethanol [11]. Recently, a variety of applications for alcohols have been suggested through chemical synthesis. The most representative field is that of the MTO (methanol to olefin) process for synthesizing olefins from methanol. Figure 10.2 shows a simplified process flow diagram commercialized by UOP [12, 13]. 2CH3 OH ! CH3 OCH3 þ H2 O ! C2 H4 þ 2H2 O

ð10:1Þ

Olefins, especially ethylene, are the most basic chemicals in the petrochemical industry. Diesel-related technologies are being developed to replace existing naphtha-based processes, in relation to ethylene production to prepare for rising oil prices and future declines in crude oil production. Ethylene produced through the MTO process itself is used as a raw material for various polymers, as well as in the manufacturing process of synthetic fuels [14] (MTG (methanol to gasoline) process, Eq. (10.2)). These processes have been commercialized by ExxonMobil et al. Fig. 10.3 shows the process flow for the MTG process [15, 16]. Methanol ! DMEðDimethyl EtherÞ ! Light Olefin ! C5 þ Olefin ! Paraffin=Naphthenes=AromaticsðGasolineÞ

ð10:2Þ

Alcohol, DME, and the like have merit in that the hydrogen/carbon ratio is much higher than that of conventional hydrocarbons, and thus a high hydrogen yield can be obtained. Based on this merit, various researches and developments have been made, in order to produce synthesis gas and hydrogen through a variety of industrial and chemical routes, such as those based on reforming and direct decomposition reactions.

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CO2 REMOVAL

PHASE SEPRATOR

COMPRESSOR (STG 4~5) DEMETHANIZER Flue Gas Ethylene

COMPRESSOR (STG 1~3)

Water Flue Gas

C2 SPLITTER

ACETYLENE CONVERTER

DRYER REACTOR

DEETHANIZER Ethane REGENERATOR

Propylene

O2 REMOVAL UNIT

C3 SPLITTER

DEPROPANIZER Crude MeOH

Air

(Recycle)

Propane C4´S C5+ DEBUTANIZER

Fig. 10.2 Schematic of a simplified process flow for the UOP/Hydro MTO process [13]

C2-

Light Gasoline

LPG

Finished Blending

Gasoline (